Some time ago, a former friend and roommate lent me around $2500 when I was going through a hard time financially. When he lent me this money, we had no agreement, written or otherwise, stating when or if I would pay him back, even though it was always my intention to do so, and I have paid some of the money back. Until recently, he was fine with this, but due to an unrelated issue, we're no longer friends and he's changed his attitude.

He works in a law firm that specializes in mortgage collections, and has begun sending me threatening letters demanding his money, plus interest that nearly doubles the original sum of money he lent me, and he's threatened to file a lien on the house I am renting, and get my wages and taxes garnished. I don't know much about the law, but I don't see how he can do this. Is he merely threatening me, or is any of this legal? I am in the state of Michigan, and he is from Florida.