Hi,
Please follow the below step.
Addministration -> Setup -> Financial -> G/L Determination -> Purchasing .
Mapped the GL Account in the Payment Advances feild.
Create the PO
Open the A/P Down Payment Invoice ...select the vendor with particular PO
Creat the Outgoing Payment.
After GRPO Create the AP Invoice and link the AP Down Payment Document.
Rgds
Kamlesh Naware