Bruce and Monica,
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.

Sure they can but if you have accepted the offer it is to late.

I am sure their agent explained to them that all parties had signed the contract and it was a binding contract at the price the seller accepeted. And if they refuse to sell the home at the accepted price the buyer can sue them. It could get very nasty. I say good luck to all involved!

Not if it is a signed properly executed contract

Seems that you are the buyer of the property - so are you being asked for more than in the contract? This would be OK if the contract had been signed but NOT DELIVERED yet. BINDING acceptance in Wisconsin only occurs after the contract is executed (signed) by all parties to the contract, AND DELIVERED as set forth in the contract. So, if the contract had not yet been delivered, and the seller got another offer, it is possible that the seller could ask for more money via a counter-offer, or multiple counter-offer.
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.

Any negotiation on an offer needs to be done prior to an offer being accepted.

I guess a better question is why? Depending on where your are with the contingencies, you will most likely close. If you look at the lines in your contract that talk about default you can get a better idea of what can happen if you do not close.

Bruce,
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.

Larry