Hi,
I'm not a specialist in this area..., but from the first looks that variable is assigned during "payment transactions" table-control loops (PBO/PAI of dynpro 130). Don't use it for anything, and not in EXIT_SAPMF02K_001 specifically - the exit is not called during those loops...
EXIT_SAPMF02K_001 receives Vendor Master Bank Details table T_LFBK. Instead of trying to grab variables somewhere out of the memory, use this table to get IBANs using FM READ_IBAN_EXT in the exit. An example how to get IBANs can be found in FORM iban_pbo of include MF02KFI0.
cheers
Jānis
P.s. to the lazy SAP programmer who first conceived FROM iban_pbo and then decided to call it - of all places!!! - too many performs to count, down in screen modification logic call chain... May you never get assignments other than maintaining Vendor Master till your retirement 