Page 4 of the rulebook:
Resource tokens (wood, food, metal and oil) remain on the board once produced. You may only spend resources from territories you currently control.
Money is not a resource.
No, you can't use the combat card to trade in for a resource to use at a later time. It acts like a wildcard of sort, where you can discard that card to help pay for something at that moment. So you can use it instead of paying 1 wood, 1 oil, 1 metal or 1 food for something.