I was just thinking about using the "bump" feature a lot of smartphones have for step 3. The bump would have a message pop up on their screen, to which they would reply yes or no and completing step 4.
1. something with a trigger like Bump,
2. which then pops up an Etherpad-style collaboratively-edited text field,
3. with MMO trade-dialog style 3-phase mutual-assent commit,
4. that leads to a copy being saved on both your phones, to a public-but-anonymous URL both parties can cite, and also forwarded to any other parties which either of you please--for example, your lawyers.