Business Disputes: Paid on Accident

My question involves business law in the state of Wyoming. I received an additional months paycheck after I had left the company. I did not realize it until my former boss (owner of the company) text messaged me months later and requested I write a check back to him within two weeks. Due to my boss's sketchy financial history (manipulating the system by charging for services that are no given.) I did not send a check. I assumed I would hear from the companies outsourced accountant; which was to no avail, therefore, I never sent a check. The accidental check directly deposited in September, and he emailed me today (January 2019) asking for the money again. Am I obligated to pay it back based simply on his request? What advice do you guys have to give on this situation? I believe blatantly sending the money back would be hell to prove on my taxes and cause an issue.

Thanks,
SaltySailor

Business Disputes: Paid on Accident