you need an attorney. He will add a security deed in court recor

absolutely can't allow the buyer get mortgage loan based on your property. Moreover, You have to pay more attention on tax and insurance. Anyway, just assume you are a bank. Then you will know what you should do and what the buyer is not able to do. If you follow legal step close the property to your buyer under attorney's help, buyer will not be able get loan using your property.

You say you bought the property using almost 5M cash? It seems impossible. You may have falled into somebody's eye!