first 2 payments were cash with no receipt. time for third. Can the check payment act as a receipt if written in memo?
The suggestion of my colleague to your duplicate posting contains very practical advise about the memo section of the check, a side-letter, and keeping independent data of payment on your computer.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.