necessità di raggruppare i record in base agli storni corrispondenti

Penso che questo sia solo row_number() e aggregazione:

select listagg(row_number, ',') within group (order by row_number) as row_numbers,
       loan_id, txn_entry_api_name, sum(txn_amt) as txn_amt,
       listagg(txn_entry_api_name, ',') within group (order by row_number) as txn_entry_api_name
from (select t.*,
             row_number() over (partition by loan_id, txn_entry_api_name, abs(txn_amt) order by row_number) as seqnum
      from t
     ) t
group by seqnum;