Deposit made to my account by mistake

I am a Independant contractor working for a large corporation that pays me via wire transfer. Over a month ago I received a wire transfer for work that I had invoiced them for as well as an additional $9000.00 that I just figured was a bonus. I called and tryied to get an answer but the lady at the front desk just kept tranfering me around to people who did not answer their phone. Finally I left a message that stated if it was a mistake let me know or I would assume it was a bonus and meant to be mine. A couple of days ago they called and left me a message that I needed to return the cash because they made a mistake. What do I do? What sort of trouble could I be in over this? I thought it was a bonus and treated myself to a weekend at the casino. The money is gone and I do not know what to do. I have a family and a small business. Please help....

You will need to pay it back if both you and they agree it was an error. So you may want to start saving for it now. They could even press theft charges.