The fact of the matter is the money has already been spent. You can't "give it back". It is gone. Whatever money there is in the future will be earned by future private sector workers and investors.
The government borrowed it and then took it out and spent it on prostitutes in Columbia (for one thing), and various other things ~ but it was a dedicated tax and the initial obligations remain ~ it must be spent on payments, or it must be refunded. You simply cannot write it off the books ~ or else!