Money with held because of loss of client

I was a contract employee, doing web development and design for an employer. I had been working for the company a little less than 3 months. My employer had handed me projects that were months behind deadline, and expected me to have them finished in a short amount of time, with no good resources to finish the project. I was unable to get a project finished in time, before a client decided that he wanted his money refunded by my employer. My employer then terminated me, he is now withholding one weeks pay from me, saying that I have cost him money. He says that if I want him to pay me for that week, he will deduct what I've cost him from my check, which would be more than I would get paid. Is he legally able to do this? Are there any free resources out there to help me get my money. It is very crucial for me right now, because my wife is ready to give birth to our second child any day now, we desperately need the money. Any help would be appreciated.