Make it so you can only create contracts with set currencies, otherwise people could spam messages. Don't want to let people set their messages.
Validate transactions that are X amount of Y currency. Have all major currencies in there, add as need demands in future releases.
Here's a decent initial list:
I'm just starting with things that apply to me. I'll edit the list if there's interest to see other things.
Pull list from the internet of world currency codes and use those?