Nested sp_executesql not working with output variable

Question

I am trying to call a stored procedure (with output variable) using sp_executesql but within another stored procedure. I wrote the following, but still not able to get trhough what that error means

This would be called from webservice code:

exec sp1 'obj1',@params 

Here obj and params are of nvarchar(max)

Definition of sp1 is :

Alter procedure [dbo].[sp1 ]  
    @procname nvarchar(max),  
    @params nvarchar(max)  
as   
    declare @temp varchar(15)  

    if @procname = 'obj1'  
    begin  
        set @params = @params +  ',@Newval  varchar(15) output'  
        EXEC sp_executesql @sp2,@params,@Newval=@temp OUTPUT  
    end  

Definition of sp2:

Here I am returning @Newval

Error I am getting :

Msg 102, Level 15, State 1, Line 1
Incorrect syntax near ','.

Also in 2 in place of exec statement , I have tried following:

 EXEC sp_executesql @sp2, @params, @temp OUTPUT; 

Results in the same error.

set @sql='sp2,' + ' @params ' + ',@params,@temp OUTPUT'
EXEC sp_executesql (@sql)

Also results in the same error.

I need this dynamic selection of stored procedures in sp1 and params is a nvarchar(max) string of parameters and their values, some of them are varchar and are embedded in ''value'' format but this string is fine as I am able to call the underlying sp2 with this.

Additional info, it it helps.

EXEC sp_executesql @sp2,@params,@Newval=@temp OUTPUT  

in this @params is combination of keys and vlaue pairs for the final sp. something like :
'@key1="a",@key2="b"'
and so on, I can not predefined the @params but it is dynamic and it is working fine when I run it with

exec (@sql)     

Format while whole of the name, params are embedded in the @sql

Answer 1

If @params='' or NULL then your , before @Newval is irrelevant. I suggest you to check:

IF NULLIF(@params,'') IS NULL or @params IS NULL
SET @params = '@Newval  varchar(15) output'
ELSE 
SET @params = @params + ',@Newval  varchar(15) output'

You are passing @sp2 maybe you need this:

ALTER PROCEDURE [dbo].[sp1]  
    @procname nvarchar(max),  
    @params nvarchar(max)  
AS   
    DECLARE @temp varchar(15)

    IF @procname = 'obj1'  
    BEGIN  
        SET @params = @params +  ',@Newval  varchar(15) output'  
        EXEC sp_executesql N'EXEC sp2 @someparam1, @someparam2, @Newval varchar(15) OUTPUT', @params, @someparam1 = 1, @someparam2 = 2, @Newval=@temp OUTPUT  
    END  

EDIT

Working example:

USE [AdventureWorks]
GO

DECLARE @procname nvarchar(max) = 'EXEC [dbo].[uspGetWhereUsedProductID] @StartProductID, @CheckDate',  
        @params nvarchar(max) = '@StartProductID int, @CheckDate date'

EXEC sp_executesql @procname, @params, @StartProductID = 1, @CheckDate = '2015-10-17'
GO