1. You can buy a mortgaged house, and your sales contract is of course valid. Because although his house was mortgaged to the bank, the ownership of the house has not been transferred and he has the right to sell it to others. But although you can buy a house, the house cannot be transferred. Even if the house is transferred, the bank can pursue it and claim that the transfer is invalid. (The rights of the mortgagee should be protected in the security interest)

2, you bought a house, claiming that the mortgage loan set up on the house has nothing to do with you. This is invalid, because once he can't repay the bank loan, the bank will recover the loss from the collateral (house), usually auctioning the house and taking it back to the bank. (For example, if he owes the bank 20,000 yuan and the bank auctions 654.38+million yuan, the bank will return the remaining 80,000 yuan to him. Of course, you can also help him pay back the money, so that the house will not be auctioned. Therefore, in any case, the effectiveness of your agreement is very weak.

3. Houses with priority mortgage can be bought and sold, but cannot be transferred. Your agreement is valid between you, but not for the bank.

My suggestion: If you really like the house, you can start with the sales contract and draw up an agreement as you said. Well, in this case, you should pay for the loss anyway. Also, you don't have to pay all the purchase price first, and then pay the rest after he has repaid the loan and transferred the ownership. Then in this case, your rights and interests will be effectively protected!