Letter agreeing to pay amount owed?

I need to write a letter that my boss can sign and states that he will pay me the money he owes me. I was supposed to get paid the amount of $7,000 by the 5th of this month, but I couldn't get a hold of him. I had to take action and go look for him at his house. He had been hiding and explained to me that he was short and only had $2,000, so he gambled it hoping he would get the full amount. How could I write a letter that he agrees to pay me by a certain date?