Why dont they pay for it by having the fed print money. That is basically what they do for everything else
Really, think about it. Lets pretend you started printing money on your laser printer, your own style not counterfeiting, and people started taking it, you go into a bar, pay your mortgage buy some stocks, its honored.
What are you going to do next?
Also what are you going take in exchange for your currency? Your own garbage currency or goods and services with a trade deficit?