Bapi for VL02N PGI which hits FI document to be posted

Hi Experts ,

I have requirment of deffered date posting ,

where basically i have to check for the payterm of SD(dilvery document VL02N) and FI document.

depending on some condition on Payterms , i have to change the Posting date of FI document .

I have written my code in user exit of vl02n but the posting date is not changing .

Please help in finding the solution or kindly suggest me any BAPI /FM to change the posting date

while Doing the PGI of SD(dilvery document VL02N) document.

Any help will be great pointer for me.

thanks and regards.

Prashant.