Set out a tower you don't have fully upgraded. Press unlock all upgrades, then exit. Go back and you should be on a loading screen. You can now sell any tower an infinite amount of times. Then when you are ready press the info button to play.
Normally when you lose the game asks you to pay Monkey Money or to retry but if you exit the game and then restart the game the "Resume Game" option will stay active allowing you to rethink you strategy this helps on the extreme maps.