listagg to rows in redshift

Question

I need to transform a list to rows to be added later, the list comes from rows in a grouped select

select
    retail ,
    listagg( distinct trn.header_id::varchar||'_'||trn.total_sale::varchar,',') as "total_sale"
from 
    ventas.mw_mdh_transactions trn
group by 
    retail

I need to read the list and add it up in the same query. All this in REDSHIFT.

for example this ...

select
    retail ,
    select z from unnest(listagg( distinct trn.header_id::varchar||'_'||trn.total_sale::varchar,',')) z as "total_sale"
from 
    ventas.mw_mdh_transactions trn
group by 
    retail