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. :)
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.