Dear,
Actually ,we will maintain Advance payment amount for Customer .If maintain AD payment for customer lets say 1000 rs then if raise sales order for 500 rs then the remain amount will be there ( F-29 ) to allow for creation of Sales Order for that customer ( 500 rs ) .So,first sales order 500 rs and second sales order 500 rs ,now you can see in F-29 then there will no amount for that customer .IN F-39 ,you can clear twice or else single time also .
If you dont mind,can you share small example regarding your requirement ( If my understanding is not clear ) .
Thanks,
Naren