Final Update:
First of all- thank you all for your extremely valuable and interesting input! This has been a very interesting situation for me, and I really enjoyed and appreciated everyone’s point of view.
In the end, the closing agent has let me know that a new closing statement will be drafted with the MF credit removed. The issue is 100% that a monetary amount cannot be added to a payable after the contract has been signed by both parties and has passed ROFR. If the sellers had requested that this credit be a part of the final amount prior to contract signing, it would have been fine- but, since this was an amount outside of any contracted obligations, they simply cannot ask for it “after the fact”.
So- lesson learned. If you are selling and want this credit, please be sure to mention it prior to contracts being generated so that it can be explicitly spelled out as part of the sale.