Residual Payment ABAP Logic

Hello Gurus!!!!

Do you know how I can obtain the amount of a residual payment in a Z program? For example, a have an invoice of 10,000 USD and I execute a residual payment of 3,600 USD. There is only left 6,400 for payment. How can I obtain the value of 3,600 searching in tables BSEG, BSAK, BSIK? Using logical database KDF complicates a little bit the solution.

Thanks!!!

Fernando.