OK, to get lots of cash this is what you have to do. (this cheat only work on tunnel, north Africa and Canada with the good guys, and Peru, Uganda, Canada, Columbia and Chechnya with the bad guys.)
Select the heavy gunner, select heavy armor, then get as much as you can (try and get to 0 cash.), then kill the VIP or hostages you are trying to protect, hold or escort.Then after the mission has restarted, sell everything except the armor.
Repeat until satisfied.