Partial Advance Clearing

Hi,

I placed and order with vendor for specific goods that totals up to 2000. vendor asks for 50% advance payment. which has been posted in SAP with special gl. Now we received partial invoice from vendor of 1000 only. what we want at this level is to adjust partial advance while making payment of this invoice. following will be the posting

GR/IR Debit 1000

Vendor Credit 1000

At the time of payment

Vendor Debit          1000

Advance Credit        500

Bank      Credit        500

Now in vendor account there will be 500 advance still remaining. Is there any way to execute this process in SAP? if note what is the reason?