The developer handed over the house to the property.

Your understanding may be a little wrong. In this case, there will be no contract assignment and it will be invalid. Only when the developer entrusts the property company, the property company has the right to handle the formalities of handing over the property as an agent. But in general, it will not appear, because many times the owner has doubts about the acceptance and needs the developer to negotiate and solve it. So the contract assignment you mentioned is invalid, and it can only be a power of attorney and an agency contract. The legal procedure is to find a notary authorization from the developer and then sign an agency contract with the property company.