That is a very controversial area. The Federal Reserve has even thrown in its opinion, which is basically that a check is a negotiable instrument with the right of deposit aside from conditions written on the check. Legal research based on both statutory and case law precedent in your jurisdiction would be required.
Opinions based on such research are reserved for members of the bar. I would not rely on opinions expressed in a public credit forum.
I would consult an atty if you are planning to make a conditional endorsement and attempt to enforce its provisions.
