Partial release of Security Deposit (R507)

Hi Experts ,

We have a requirement of partial release of security amount ... means at the time of move-out we have to release Security amount equal to final balance of the customer. Example If customer have Security of 5000 $ and final Invoice amount or Net balance is 3500 $ then only 3500 $ needs to be released not whole SD amount.

We found out a event R507 and make Z copy of the FM as well but in 'FKK_FUNC_MODULE_DETERMINE' both Event as well as FM is showing but logic inside the FM is not triggering.

So tell me

1. do we have any other way to do it .
2. or we are doing something wrong while creating the customized Z FM.

Please find the screen shots.