Should detachment, and letting go, be an appropriate response when you are owed money?

I'm trying to adhere to buddhist principles in my life wherever I can. I understand that it's really the best approach in just about every situation you can find yourself in. However, right now I'm owed a not insubstantial amount of money from one of my clients who is refusing to take my phone calls, or answer emails to discuss the matter. Honestly, it burns me up inside and when I think about having to deal with it I feel angry and resentful which I know is not a good way to live. From my understanding I should really just let go of the matter completely, and calmly wait for the universe to resolve this for me, one way or another. If I receive the money, then all is well, and if I don't receive the money, all is still well. From a Buddhist perspective, at what point should I give up and move on?