GRIR clearing through BDC

Hi,

We have a very peculiar situation, whereby in the GR/IR Clearing Account, the sort key was mentioned as 001 (Posting Date), instead of 014 - Purchase Order + Item. The result is that in the Assignment Feild instead of Purchase Order No. + Item, the Posting Dates have got populated. my client using TC F-03 for GRIR clearing, now my problem is i am unable to get logic for developing BDC.

Rgds,

Venki