1. In this case, don't help him repay the loan, and don't use your down payment to repay the loan. If he repays the loan and finally doesn't sell it, you can't find a reason.
2. The formal intermediary will not let you pay the down payment to the landlord to repay the loan. Under normal circumstances, the house will find its own funds to repay the loan. Don't pay it back, or it will be too late for you to regret it.