Legal advice, urgent!

First of all, it is clear that Li Si is Zhang San's guarantor and Zhang San's house is collateral, both of which are guarantees for the bank. In the case of Zhang San's inability to repay, the bank has the right to dispose of the collateral and ask the guarantor to repay it on his behalf. As for the agreement between Li Si and Zhang San, it cannot be against the mortgage of the bank, that is, the agreement between Zhang and Li is valid only if the bank gives up the mortgage of driving. In order to protect his rights and interests, Li Si can negotiate with Zhang San to handle the house transfer formalities in advance with the consent of the bank. After the house is registered, he can confront Zhang San's other creditors. But Zhang San has no right to cancel the bank's mortgage. I hope I can help you.