The main risk is that his house is mortgaged in the bank, and the certificate has not yet been issued. So you can't buy a house now. In other words, after you pay him, you have to wait two years before he can transfer the ownership certificate to you. Can you guarantee that he will fully cooperate with you in two years? You can't guarantee whether he will go back on his word after two years, because the current house price is uncertain what it will be in two years. If it goes up much, he would rather break the contract than sell you, and you will suffer.
If you must buy this house, you must first check whether he owns the authenticity of this house. After issuing the certificate, the other party is required to cooperate with the relevant agreements of the transfer, as well as the liabilities for breach of contract in all aspects, etc. But such a contract doesn't make much sense.
You'd better buy a house in the sales office. Why take the risk? It's good to meet the right person in an argument. Think for yourself.