If you cash the check you run the risk of creating a legal issue around the authority to do so. While the check was issued, it was given to you with stipulations. You don't have to agree to those stipulations, but they create ambiguity as to whether you have received authority to negotiate the instrument. Cashing it would likely create more hassle than it is worth.
To counter this situation, my strategy would be to send the customer a letter sent via certified mail with return receipt (so you can prove he got it) stating that you received his payment, that is was an overpayment, that only $__ was due for the following services, and that you will deposit the check to cover those services and refund the balance, unless you hear from him with a seven days (or whatever time period you choose).
If the person responds negatively, then return the check and demand payment or turn him over to collections. If he doesn't respond, document that fact and cash the check and return the balance to him.
Please let me know, if you have follow-up questions.
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In legal matters, time is of the essence. As such, you should contact an experienced lawyer right away to protect your legal rights!