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?
Imagine a monkey came to your house and grabbed something of value and climbed a tree. You tell the monkey "please give it back to me! Be a good monkey!". But it doesn't respond. What would your plan of action be? Start cursing and hating the monkey? No! You would try to lure the monkey with a banana or something. If that's not successful, you might call a wild life officer. You have to use your wisdom instead of getting all emotional. And if you still can't get it back, you don't wish for the universe to resolve it. ex: Wishing for the monkey to be struck by lightning. :) You just let it go and credit it towards experience or a lesson learnt and see what you can do to prevent the same from happening in the future.