The spirit of the contract is that you won’t try to change what you agreed to.

In real life, people change their minds all the time. YOu have several portions of the contract that give you the right to cancel. 10 days to check out the house ( many people cancel on day 9 because of the changing market conditions, but they give the reason as “the condition of the house” or “the school district is unacceptable”).
If the appraisal comes in low, you can cancel with no questions asked.
Sometimes it is best to buy your way out. Ask the sellers if they will accept $250 to release you today.

No, you already have a contract, you can not negotiate it now, you already did that. The markets are stable anyway, you should not see a difference in value.