Retention - Seperate GL Account

Hi,

A billing document (invoice) is generated following the PR-PO process. The amount to be paid to the Vendor is 90% of the invoice amount immediately and the balance amount of 10% is retention and is to be paid only after satisfactory completion of the service, say 6 months. A payment term is created where the invoice is broken into separate line items - 90% and 10% and accordingly posted in Vendor account.

Now the case is that, the amount is not paid thru SAP. The amount is passed on to the legacy system from where the payment takes place.

The requirement is, the retention amount instead of going to Vendor account, need to go to a separate GL account. How can this be achieved in SAP?

Please revert.

Thanks,

M.R.Srinivas