You will get a money bag everytime you manage to level a unit to 3* on your field. It happens automatically after you select the relic. Also, if you already have a 3* unit on your field at the time you select the relic, you will get the bags instantly. Keep in mind though, that the money you get from the bags depends on how many units with the same name as your 3* unit are posessed by your opponents (the more popular the unit, the more money is in the bag)