The purchase contract can only show that both of you have reached an agreement on the meaning and price of the house. Before the house is sold to a bona fide third party, you can ask them to continue to perform the contract-transfer the ownership. If they sell the house to a third party who doesn't know your trading relationship, then you can only claim liquidated damages, but not claim the ownership of the house on the grounds of contract.
As for the contractual behavior, as long as Lao Zhang is a person with full capacity, that is, he is not mentally ill, oppressed or deceived; Then the contract he signed is protected and bound by law. Without his wife's authorization, even if his wife denies it, you can directly sue Lao Zhang to the court for compensation.
Husband and wife are agents of each other. They don't need each other's consent when dealing with the expenses of daily necessities. However, when it comes to major property such as houses, both parties need to agree unanimously. Except for those that a third party thinks one party has the right to handle, such as Lao Zhang, you and the intermediary need to prove it.
Having said that, you should remember that the ownership of the house has not changed. You just signed a house sales contract, which is true and valid and protected by law. You can only urge him to fulfill the contract and transfer the ownership. If they just don't come in, there's nothing you can do about them. We can only fight them through breach of contract, that is, we advocate liquidated damages. As long as there is no deadline for performance, such as 30 days, you can sue.
As for whether Lao Zhang has no right to act as an agent, don't worry about him. Even if he has no right to dispose of it, the creditor's rights behavior does not need to have the legal right to dispose of it. Even if Lao Zhang has no right to dispose of husband and wife's property, he can sign a contract. If he fails to perform, he will be liable for breach of contract, because he is only responsible for you, and his contractual responsibilities need to be guaranteed by their husband and wife's property, so you don't have to worry about what his wife says: he has no right to represent.