Population of XREF2-Reference Key 2 for a Accounting Document

Hi Gurus,

I have a requirement while posting an accounting document by clearing the Customer open item.

My requirement is Populating the BSEG-XREF2 with customer tolerance Group (KNB1-TOGRU) of the Customer.

I tried implementing the logic in the substitution rule but it is not working. At the time of the subsitution rule code, there is no value in the Field BSEG-KUNNR and i am not able to pull the Customer Tolerance Group.

Can you please tell me are there any user exist or enhancement points to Populate BSEG-XREF2 while posting an accounting Document.