I'm not a lawyer and this is not legal advice: disputing debts is not by itself fraud, even if you end up losing. If you have some "colorable claim" (i.e. some basis in law and fact to think that a court might plausibly rule in your favor), then you are in your rights to test it in court. But don't be surprised if upon losing, you are forced to pay interest and/or other damages accrued due to the delay.