How to maintain salesgroup a partial payment

Dear all

We would like to know how to maintain a salesgroup when we book a partial payment in the FBL5N.

The situation now is that a fictiv line is booked without a salesgroup we would like to maintain the salesgroup when a partial payment is booked on a invoice and when its booked on the debetors card without linking to a invoice.

thanks for your help in advanced

With kind regards,

Dennis Kristoffers