A rent-back agreement sets a daily or weekly occupation cost, a hard move-out date and what happens if the seller does not leave on time, and your lawyer, whose fees typically run $800 to $1,500 for a standard file, should be the one drafting it alongside the purchase contract, not after possession has already passed.
Buyers vary in how open they are to this. A buyer who is renting until their own possession date has flexibility to offer; a buyer who needs to move in immediately usually will not agree to any rent-back at all, so it changes which offers you can accept.
Joel Dyck negotiates rent-back terms as part of the offer itself when a seller needs more time. Get your hand-reviewed valuation from Joel Dyck.