It's simple accounting. Net money in must be more than or equal to net money out when adjusting for any temporal changes in the face value of the assets. If you use more than you produce, either you are going to have to pay in the future, or in the case of the government, you can die and let your children pay.

There's no law of conservation of money. It can be pretty trivially created and destroyed. It may not be a good idea, but it's entirely doable.