A general rule of thumb is anything can happen in a real estate transaction. That being said it is not common that the request comes in, and even less that is is successful. The buyer has a right to ask for anything at any time, and the seller has a right to say yes or no at any time, however after the removal of contingencies if the seller has no incentive to say yes it often does not happen. If there is an incentive, I have seen it happen. So, if the seller did not reveal a material fact, something that is important that affects the value, then they have been known to renegotiate rather than face potential legal problems. If there is a precipitous change in the value of the house, and a buyer backing out would be more costly than changing the price I have seen it happen. When you ask if it is ethical, I think that is harder question to answer. Is it ethical for a seller to conceal something important? Is it unethical to say the house has lot 10% in value since I made the offer and you only hold a 3% deposit so renegotiate or you will lose 7%? Is it ethical to say I do not want to pay what I said I would so help me out? It depends on the circumstance?