User exit for transaction type in FBB1

Hi All,

We have a requirement which allows the users to overwrite the transaction type and save it with different TType when they doing the entry manually via FBB1. For a particular account a transaction type is maintained in GGB1 say Z28. So for those accounts TType is automatically coming as Z28 even if we change that value also its not saving that, still showing Z28 only.

Can you please suggest how to implement the above requirement using User exit.

Thanks,

Spandana