Ask HN: How do I make them pay up?

I had a contract with a client and had a sub-contractor. Client pays me, I take a cut and pay my sub-contractor - nothing strange here. My client wanted to pay my sub contractor directly, so I worked it out with the sub contractor to pay me a "finder's fee". Basically, the same end result. It's has been going well for several months but now the sub contractor is trying to renege and not pay. What should I do? I have been open with everyone and all parties involved know that I am making money on this deal. I could go to the client and let them know that sub contractor is not paying me but that feels unprofessional and could potential hurt the sub contractor's reputation i.e. lose-lose for everyone involved. I am extremely disappointed by the sub contractor's unprofessional and unethical attitude. Any suggestions on how I can handle this situation?