Recently, at the request of my tech support contractor, I renewed my service contract with him. You did not commit a crime and you probably did not violate the law.
Anytime criminal prosecution is threatened, it is wise to consult with an attorney, whether to ease your concerns about your own actions or to determine if the person threatening is engaging in extortion.
And, seeing an attorney early may aid you in clearing up this matter before it blows up into something more complicated and expensive.
Now, I learn that he immediately tried to cash the check and it was refused by the bank! But, in general, it is not a crime to post-date a check.
This article discusses general laws about checks, post-dating, and the parties’ rights.
(c) Sharing quarters or office space, or having a common waiting room with a practicing attorney or a lender.