Let us imagine for a moment that you live in a nation, and you request some benefits. The government tells you the cost is $500 for the benefits, and you pay $500 in taxes for the benefits. You get your benefits. No problems. Well worth the money you paid for them. No complaints.
Then when you have your benefits, the government shows up at your door, apologizes profusely, explains that it underestimated the real cost of providing you with the benefits it agreed to provide and hands you a bill for an additional sum it had to borrow in order to provide the services, plus interest since the start of your use of the benefits. Do you pay it? Of course not. You, the citizen, entered into a verbal contract with the government to provide you with your benefits at the taxes agreed to by all parties prior to the transaction.
If the government has miscalculated the cost of meeting the agreed-to obligation to the citizen, is the citizen obligated to cover the shortfall? No. The government is responsible for the error, and if it cannot meet it's agreed-to obligations for the agreed-upon price, it should declare bankruptcy, go out of business, and make way for a new government with better fiscal sense.
Yes, the actual dollar amounts are out-of-date, but the principle is still the same.