Hi
Maybe you can see the logic of "documentary payments" (SD-FT-LOC) in this print file http://help.sap.com/printdocu/core/print46c/en/data/pdf/SDFTLOC/SDFTLOC.pdf
It's a short document (around 30 pages). You avoid customer tables. Check if it is not so expensive in clerical work.
I hope this helps you
Regards
Eduardo