Rention Amount by Customer on Lease - Payment Terms

Hello All,

Can anybody tell me how to do the below scenario in RE-FX.

1. Our company enters in to residential lase contract with Govt., Company, at that time 5% of the Lease amount will be held by the Govt., company as Retention money and it is paying that back only on the date of next renewal (as a guarantee for better service).

How to answer the above requirement in RE-Fx. I tried to use payment terms of FI , but in RECN transaction when i tried to use this payment term,system showing a message to do this using the contract account sheet. But i am not able to find any option in the contract account sheet to create installment or change of due date.

Thanks,

Bala.