EXIT_SAPMF02K_001 to check IBAN

Dear SAP Gurus,

I get a problem when I want to check IBAN Value in transaction code FK02 since I don't want any double IBAN when end user modify the payment transaction screen in FK02.

I am using EXIT_SAPMF02K_001 (INCLUDE ZXF05U01) to check it.

Actually I get the variable for IBAN, that is IBAN_VAL when reading FK02 program using breakpoint.

But How to bring this value to this function module (ZXF05U01) ?

I mean how this function module (ZXF05U01) knows this variable (IBAN_VAL) ?

Can anybody help ?

Regards,

Daniel