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.

I would say,

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.

We live in social days. Add a FB/Twitter sharing "Just got an investissement from X"/"Just invested in Y"

It has to be confirmed, so wait till it gets retweeted/liked a few times, and not on an orphaned platform.

Build in SHA256 signing and you have something rock solid.

I would say it would be "easier" (in a way) to just back each of the "assent" phases (you agree, they confirm, you confirm) with an equivalent (automatic in-the-background) GPG signing. Then, what gets sent around is your signature of their signature of your signature of the final plaintext. :)

And a RFC. And an API.

I just made a little website based on the HDP, check it out: http://www.hdplist.com