Go to appdata folder on your computer
Go to "Local" folder
Go to the "Teardown" folder
Go to "savegame.xml"
Open with notepad or other applications
Scroll to <cash value ="number">
Change the number in Cash Value to what ever number you want!

Open teardown
Go to options
Go to game
Enable Mission Skipping
Close Settings
Go to mission of your choice
Press "Skip Mission"

Do the mission skipping on all levels
then you should unlock every sandbox level!