Buyer delaying closing

[Chris M.]

The buyer for my house took 45 days and still hasn't closed. Now he's saying his bank can't finance the mortgage because he forgot to tell them about his student loans.  My agent says she has a different mortgage company that can still close the deal, but it will take another 3 weeks. So they are asking me to sign an extension. Its costing me $1000/mo just to carry the house.  Should I just agree to give them another 3 weeks for free or ask for a penalty?