Validations on tx FCH5.

Hi! Im having some trouble to make validations at selection screen on tx FCH5.

Specifically, i need to check that the user's input (short key to house bank (HBKID) and the id for account details (HKTID) ) matches with some information about the payment document.

The thing is that i allready try to do this by using field exits. But it doesnt work. In some cases, the validation fails.

Is there anyone who knows a better way to make validations on FCH5 tx screen than using field exits?

thank you very much for your invaluable help.