Hi Sarat,
If I understand you correctly, you want to decide at the time of Payment run, which House bank you want to pay from.. if that is the case, you can use the following transaction.
S_ALR_87001486 - Enter Available amounts for the Payment Program.
In FBZP, you should maintain available amount as 0 and at the time of payment run, you should enter amount available in this transaction just before the payment run depending upon the funds availability, then system will pick up that bank only against which you have maintained funds. The logic is that system will still check the ranking but since no amount will be available in Ranking 1, it will look for amounts in Ranking 2 and select that house bank for payment.
Hope this helps you.
Regards,
Kavita