[throwaway2019Z]

Did you sign a contract promising a certain level of work? Do you have documented communications between the two of you whereby you stated that the job "was outside of my knowledge and did not feel comfortable"?

In all likelihood, he sounds like a scummy client trying to rip you off. Unless you agreed to a 3rd party audit prior to starting work, it is also unlikely that he has the right to withhold your last paycheck.