Posting a vendor buy-out credit

We're switching copier vendors. Our new copier vendor (B) provided a buy-out credit for the residual of our old vendor's (A) leases. I need to post this credit from one vendor to use to pay-down the leases from Vendor A over the next 6 months. What's the best way to post this credit from Vendor A so that it's applied to Vendor B for future expenses?