rent back briefly is differnt from formal rental agreement

First, check with your RE agent and local regulations.

If only rent back briefly, PIPT is the normal monthly charge. Make sure you have deposit in escrow.

If the seller becomes your tenant after closing, you charge by the market rent.

It seems your contract has been ratified. If the seller does not agree with your counter rent back addedum, you still have a valid sales contract and can take legal actiions for non performing.

Let your RE agent work with the seller's agent. That's how they get paid.