Money is the only resource that is stockpiled. On your stats bar, you see two numbers, something like 99765(+60). The first of these two numbers is your reserve, your stockpiled Money. The second number is how much Money you are earning every minute.When you pay to deploy new regiments or buildings, the monetary cost is deducted from your reserve. While your per-minute Money income can be negative, you can never have less than zero in your reserve. If, however, your reserve is at zero, and you have a negative income, things will start to go badly for you. For instance, any units that are in supply are supplied much more slowly than normal (units that are not in supply have their health slowly drain away until they disappear altogether). Also, you cannot construct new buildings or repair the ones you have when you have a negative income.



Source: Axis and Allies Official Manual