Proof of Communication
This sounds like "fair exchange," the subject of many good research papers. In general you need a third party to give any security guarantees, but "optimistic fair exchange" involves the third party only when one of the parties tries to cheat (i.e., when both play honestly there is no involvement from the third party).

Incidentally, Diffie-Hellman is most certainly not fair: whoever sends first lets the other party compute the key before the original sender can.