XPath Append '-' to Element Values When Sibling Matches String

Question

I am trying to create an XPath with v1.0 that returns a list of all amount elements Amt[@Ccy="EUR"]. However, it should be noted that if the sibling element of this element CdtDbtInd has the value DBIT, a - should be appended to the beginning of the string. If this is not the case, the string should not be manipulated.

I already managed to append a char at the beginning of the string like so:

//BkToCstmrStmt/Stmt/Ntry[Amt[@Ccy="EUR"]/../CdtDbtInd='DBIT']/concat("-", Amt[@Ccy="EUR"]/text())

This transforms my value like this:

// from
1.00

// to 
-1.00

But this only returns me the values where this is the case ...

Now I somehow need a way that allows me to build this into an if then else condition so i can get the values of every //BkToCstmrStmt/Stmt/Ntry/Amt[@Ccy='EUR'] tag.

for example:

IF "//BkToCstmrStmt/Stmt/Ntry[Amt[@Ccy="EUR"]/../CdtDbtInd='DBIT']" THEN
    concat("-", Amt[@Ccy="EUR"]/text()
ELSE
    Amt[@Ccy="EUR"]/text()

According to this post i tried it with the Becker's Method:

concat(
    substring(
        concat("-", //BkToCstmrStmt/Stmt/Ntry[Amt[@Ccy="EUR"]/../CdtDbtInd='DBIT']/Amt),
        1,
        number(
            //BkToCstmrStmt/Stmt/Ntry[Amt[@Ccy="EUR"]/../CdtDbtInd='DBIT']
        )
        * string-length(concat("-", //BkToCstmrStmt/Stmt/Ntry[Amt[@Ccy="EUR"]/../CdtDbtInd='DBIT']/Amt))
    ),
    substring(
        //BkToCstmrStmt/Stmt/Ntry/Amt[@Ccy="EUR"]/text()
        1,
        number(not(
            //BkToCstmrStmt/Stmt/Ntry[Amt[@Ccy="EUR"]/../CdtDbtInd='DBIT']
        ))
        * string-length(//BkToCstmrStmt/Stmt/Ntry/Amt[@Ccy="EUR"]/text())
    )
)

But this throws the error: Error: Required cardinality of first argument of string-length() is one or zero

Heres my XML File:

<?xml version="1.0" encoding="UTF-8"?>
<Document xmlns="urn:iso:std:iso:20022:tech:xsd:camt.053.001.02" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:iso:std:iso:20022:tech:xsd:camt.053.001.02 camt.053.001.02.xsd">

<BkToCstmrStmt>
  <GrpHdr>
    <MsgId>053D2013-12-27T22:05:03.0N130000005</MsgId>
    <CreDtTm>2013-12-27T22:04:52.0+01:00</CreDtTm>
    <MsgPgntn>
      <PgNb>1</PgNb>
      <LastPgInd>true</LastPgInd>
    </MsgPgntn>
  </GrpHdr>
  <Stmt>
    <Id>0352C5320131227220503</Id>
    <ElctrncSeqNb>130000005</ElctrncSeqNb>
    <CreDtTm>2013-12-27T22:04:52.0+01:00</CreDtTm>
    <Acct>
      <Id>
        <IBAN>DE58740618130100033626</IBAN>
      </Id>
      <Ccy>EUR</Ccy>
      <Ownr>
        <Nm>Testkonto Nummer 2</Nm>
      </Ownr>
      <Svcr>
        <FinInstnId>
          <BIC>GENODEF1PFK</BIC>
          <Nm>VR-Bank Rottal-Inn eG</Nm>
          <Othr>
            <Id>DE 129267947</Id>
            <Issr>UmsStId</Issr>
          </Othr>
        </FinInstnId>
      </Svcr>
    </Acct>
    <Bal>
      <Tp>
        <CdOrPrtry>
          <Cd>PRCD</Cd>
        </CdOrPrtry>
      </Tp>
      <Amt Ccy="EUR">8.50</Amt>
      <CdtDbtInd>CRDT</CdtDbtInd>
      <Dt>
        <Dt>2013-12-27</Dt>
      </Dt>
    </Bal>
    <Bal>
      <Tp>
        <CdOrPrtry>
          <Cd>CLBD</Cd>
        </CdOrPrtry>
      </Tp>
      <Amt Ccy="EUR">18.50</Amt>
      <CdtDbtInd>CRDT</CdtDbtInd>
      <Dt>
        <Dt>2013-12-27</Dt>
      </Dt>
    </Bal>
    <Ntry>
      <Amt Ccy="EUR">2.00</Amt>
      <CdtDbtInd>CRDT</CdtDbtInd>
      <Sts>BOOK</Sts>
      <BookgDt>
        <Dt>2013-12-27</Dt>
      </BookgDt>
      <ValDt>
        <Dt>2013-12-27</Dt>
      </ValDt>
      <AcctSvcrRef>2013122710583450000</AcctSvcrRef>
      <BkTxCd/>
      <NtryDtls>
        <TxDtls>
          <BkTxCd>
            <Prtry>
              <Cd>NMSC+051</Cd>
              <Issr>ZKA</Issr>
            </Prtry>
          </BkTxCd>
          <RltdPties>
            <Dbtr>
              <Nm>Testkonto Nummer 1</Nm>
            </Dbtr>
            <DbtrAcct>
              <Id>
                <Othr>
                  <Id>  740618130000033626</Id>
                  <SchmeNm>
                    <Cd>BBAN</Cd>
                  </SchmeNm>
                </Othr>
              </Id>
            </DbtrAcct>
          </RltdPties>
          <RmtInf>
            <Ustrd>TEST BERWEISUNG MITTELS BLZUND KONTONUMMER - DTA</Ustrd>
          </RmtInf>
        </TxDtls>
      </NtryDtls>
    </Ntry>
    <Ntry>
      <Amt Ccy="EUR">3.00</Amt>
      <CdtDbtInd>CRDT</CdtDbtInd>
      <Sts>BOOK</Sts>
      <BookgDt>
        <Dt>2013-12-27</Dt>
      </BookgDt>
      <ValDt>
        <Dt>2013-12-27</Dt>
      </ValDt>
      <AcctSvcrRef>2013122710583670000</AcctSvcrRef>
      <BkTxCd/>
      <NtryDtls>
        <TxDtls>
          <Refs>
            <EndToEndId>NOTPROVIDED</EndToEndId>
          </Refs>
          <BkTxCd>
            <Prtry>
              <Cd>NMSC+051</Cd>
              <Issr>ZKA</Issr>
            </Prtry>
          </BkTxCd>
          <RltdPties>
            <Dbtr>
              <Nm>Testkonto Nummer 1</Nm>
            </Dbtr>
            <DbtrAcct>
              <Id>
                <IBAN>DE14740618130000033626</IBAN>
              </Id>
            </DbtrAcct>
            <UltmtDbtr>
              <Nm>keine Information vorhanden</Nm>
            </UltmtDbtr>
            <Cdtr>
              <Nm>Josef Schliffenbacher</Nm>
            </Cdtr>
            <CdtrAcct>
              <Id>
                <IBAN>DE58740618130100033626</IBAN>
              </Id>
            </CdtrAcct>
            <UltmtCdtr>
              <Nm>keine Information vorhanden</Nm>
            </UltmtCdtr>
          </RltdPties>
          <RltdAgts>
            <DbtrAgt>
              <FinInstnId>
                <BIC>GENODEF1PFK</BIC>
              </FinInstnId>
            </DbtrAgt>
          </RltdAgts>
          <RmtInf>
            <Ustrd>Test+berweisung mit BIC und IBAN SEPA IBAN: DE14740618130000033626 BIC: GENODEF1PFK</Ustrd>
          </RmtInf>
        </TxDtls>
      </NtryDtls>
    </Ntry>
    <Ntry>
      <Amt Ccy="EUR">1.00</Amt>
      <CdtDbtInd>DBIT</CdtDbtInd>
      <Sts>BOOK</Sts>
      <BookgDt>
        <Dt>2013-12-27</Dt>
      </BookgDt>
      <ValDt>
        <Dt>2013-12-27</Dt>
      </ValDt>
      <AcctSvcrRef>2013122711085260000</AcctSvcrRef>
      <BkTxCd/>
      <NtryDtls>
        <TxDtls>
          <BkTxCd>
            <Prtry>
              <Cd>NTRF+020</Cd>
              <Issr>ZKA</Issr>
            </Prtry>
          </BkTxCd>
          <RltdPties>
            <Cdtr>
              <Nm>Testkonto Nummer 1</Nm>
            </Cdtr>
            <CdtrAcct>
              <Id>
                <Othr>
                  <Id>  740618130000033626</Id>
                  <SchmeNm>
                    <Cd>BBAN</Cd>
                  </SchmeNm>
                </Othr>
              </Id>
            </CdtrAcct>
          </RltdPties>
          <RmtInf>
            <Ustrd>RÜCKBUCHUNG</Ustrd>
          </RmtInf>
        </TxDtls>
      </NtryDtls>
    </Ntry>
    <Ntry>
      <Amt Ccy="EUR">2.50</Amt>
      <CdtDbtInd>CRDT</CdtDbtInd>
      <Sts>BOOK</Sts>
      <BookgDt>
        <Dt>2013-12-27</Dt>
      </BookgDt>
      <ValDt>
        <Dt>2013-12-27</Dt>
      </ValDt>
      <AcctSvcrRef>2013122712332300000</AcctSvcrRef>
      <BkTxCd/>
      <NtryDtls>
        <TxDtls>
          <Refs>
            <EndToEndId>STZV-EtE27122013-11:02-2</EndToEndId>
          </Refs>
          <BkTxCd>
            <Prtry>
              <Cd>NMSC+051</Cd>
              <Issr>ZKA</Issr>
            </Prtry>
          </BkTxCd>
          <RltdPties>
            <Dbtr>
              <Nm>Testkonto Nummer 1</Nm>
            </Dbtr>
            <DbtrAcct>
              <Id>
                <IBAN>DE14740618130000033626</IBAN>
              </Id>
            </DbtrAcct>
            <UltmtDbtr>
              <Nm>keine Information vorhanden</Nm>
            </UltmtDbtr>
            <Cdtr>
              <Nm>Testkonto Nummer 2</Nm>
            </Cdtr>
            <CdtrAcct>
              <Id>
                <IBAN>DE58740618130100033626</IBAN>
              </Id>
            </CdtrAcct>
            <UltmtCdtr>
              <Nm>Testkonto</Nm>
            </UltmtCdtr>
          </RltdPties>
          <RltdAgts>
            <DbtrAgt>
              <FinInstnId>
                <BIC>GENODEF1PFK</BIC>
              </FinInstnId>
            </DbtrAgt>
          </RltdAgts>
          <RmtInf>
            <Ustrd>Sammelueberweisung 1. Zahlung EREF: STZV-EtE27122013-11:02-2 IBAN: DE14740618130000033626 BIC: GENODEF1PFK ABWE: Testkonto</Ustrd>
          </RmtInf>
        </TxDtls>
      </NtryDtls>
    </Ntry>
    <Ntry>
      <Amt Ccy="EUR">3.50</Amt>
      <CdtDbtInd>CRDT</CdtDbtInd>
      <Sts>BOOK</Sts>
      <BookgDt>
        <Dt>2013-12-27</Dt>
      </BookgDt>
      <ValDt>
        <Dt>2013-12-27</Dt>
      </ValDt>
      <AcctSvcrRef>2013122712332300001</AcctSvcrRef>
      <BkTxCd/>
      <NtryDtls>
        <TxDtls>
          <Refs>
            <EndToEndId>STZV-EtE27122013-11:02-1</EndToEndId>
          </Refs>
          <BkTxCd>
            <Prtry>
              <Cd>NMSC+051</Cd>
              <Issr>ZKA</Issr>
            </Prtry>
          </BkTxCd>
          <RltdPties>
            <Dbtr>
              <Nm>Testkonto Nummer 1</Nm>
            </Dbtr>
            <DbtrAcct>
              <Id>
                <IBAN>DE14740618130000033626</IBAN>
              </Id>
            </DbtrAcct>
            <UltmtDbtr>
              <Nm>keine Information vorhanden</Nm>
            </UltmtDbtr>
            <Cdtr>
              <Nm>Testkonto Nummer 2</Nm>
            </Cdtr>
            <CdtrAcct>
              <Id>
                <IBAN>DE58740618130100033626</IBAN>
              </Id>
            </CdtrAcct>
            <UltmtCdtr>
              <Nm>Testkonto</Nm>
            </UltmtCdtr>
          </RltdPties>
          <RltdAgts>
            <DbtrAgt>
              <FinInstnId>
                <BIC>GENODEF1PFK</BIC>
              </FinInstnId>
            </DbtrAgt>
          </RltdAgts>
          <RmtInf>
            <Ustrd>Sammelueberwseisung 2. Zahlung EREF: STZV-EtE27122013-11:02-1 IBAN: DE14740618130000033626 BIC: GENODEF1PFK ABWE: Testkonto</Ustrd>
          </RmtInf>
        </TxDtls>
      </NtryDtls>
    </Ntry>
  </Stmt>
</BkToCstmrStmt>
</Document>

Thanks for any help with this!