Are you in the United States? I’m surprised you haven’t contacted a lawyer. Your agreement is with Upwork, they are obligated to pay you. If they get a bad check from the client, it’s their problem.

Upwork is based in California, which has a strong Attorney General. You should notify the AG immediately.

If Robin used a credit card without permission, he may be liable for criminal as well as civil damages. If he is based in Switzerland, find out which authorities are responsible. If

This whole thing sounds sketchy. Upwork can’t force you to work for nothing. They can sue you for damages and then maybe make an offer. What if you can’t get clients? What if you die? Is there a claim against the estate?