3.70: Structures: 3rd party payments

Hi all, I need guidance on following issue. In the structures table, A functionality is provided to make 3rd party payments to vendors. This functionality works well on a Purchase order. When I mention the vendor code and the G/L account on the order structure,The Vendor ledger does get updated on posting the order. The same gives an error( Amount should not be positive on the General Journal Line) Has anybody experienced the same? Solved it? The requirement is: ( On a Sales order for 100000 $, I need to pay commission of 2 % to a broker.This commision is part of the Total order price and is not to be added on the Base amount.It is to be considerd as an additional expense and is to be paid to a particular broker.) Regards, DD