You can "ask" for anything but your buyer may not be receptive. If there are some aspects of the purchase agreement that are open-ended in your favor, you may be able to require an increase but that would be rare and exceptional in most agreements.
If the contract was delivered to you - then it is a binding contract,a nd can only be changed by mutual amendment of the parties.
Good luck! If you can't get satisfaction with this, it is a good time to check with an attorney.
The problem is the offer turned into a contract as soon as you accepted it and signed it. So now the buyer has a contract for the purchase of your home. If there is a problem then you can have your contract looked at by an attorney to see if you agent gave you an out. But, this is now a contract.