Potential earnings cannot be recovered in law suits. One can only recover damages.

There was a famous case where someone accidentally damaged the electricity cable to a iron wielding company during construction work. The whole factory shut down for several hours. The factory sued for damages. The judge ruled that: Cost of wasted iron material, check. If the wielding process is interrupted the material is wasted. Cost of repairing machines, check. Machines where stopped at places they should not be. Potential business revenue during the shut down, no. The court would not consider the possibility of how would the business have conducted had some event in the past been different.

Your client is merely using the threat of a lawsuit in an attempt to have you done certain things. It sounds like you would rather forget the money than face the headache of a lawsuit. I doubt he can bring one, since he has to establish he has suffered damages; but if he does and you end up having a lawsuit anyway, you can use the opportunity to recover the service fee he owns you. The circumstances are in your favor.

_{I forgot what the name of the case was. If someone can point it out in the comments (or edit in) that would be great.}