VPRS Condition value in Intercompany Billing

Scenario:

My Client maintains VPRS Condition type in Sales Order and Inter company billing document (IV) as well.

If the sales order was created on 1st Feb (when VPRS was 100) and for some reason the Inter company billing document was not created immediately. Meanwhile the VPRS got changed to Rs. 200.

We need to maintain/have the same condition value which was there in the preceding document or the price which was there during those days.

Here is the Challenge:

We have a custom program to create billing documents. (Excluding option of using Copy controls option).

Dear Gurus would you please find out an option to achieve solution in this case.

Regards,

Ram