You've got the wrong idea here. The government "pays for" everything it buys. All the time. Forever. But it doesn't need to get money from someplace else then send it on to various vendors like you and I do. Because the US currency is sovereign. When the US sends us a refund check, for example, nobody checks the account to make sure there's enough in it before sending the check. The feds send a piece of paper with some numbers on it and the bank accepts it because...it came from the US. When the bank deposits the money in our account, a few keystrokes puts it there and doesn't take it from anywhere else. Again, there's not a big checking account somewhere that gets debited when my account is credited.
All to say, yeah, the US has to "pay for it" in that it can't force anyone to provide goods and services for free, but it doesn't "pay for it" in the sense of "has to find the money somewhere either now or later".