to change assignment field (ZUONR) in accounting document when doing PGI

Hi Gurus,

I have one requirement. I will create one sales order with the reference of contract and then i will do the delivery and I will do PostGoodsIssue(PGI).In standard at that time it creates an accounting document in background.At that time I need to change that assignment field (ZUONR) value to contract number in that Accounting document.

I treid with BAPI and Business Transaction Events like (1130,1120,1025) but it is not working.

So please help me in this issue.

Thansks & Regards,

R.P.Sastry.