For the government to borrow from itself has about as much meaning as me borrowing from myself.
Have you ever borrowed from one credit card to pay off another? It's a dumb thing to do, but many people do it all the time.
If you don't cut spending, you eventually will run out of options when you hit your credit limit. But, the federal government can raise it's own credit limit.
However, the question here isn't whether the government is acting wisely. The usual claim is that the government "stole from Social Security". They didn't. Every dollar that was borrowed is accounted for, and is earning interest.
And the real issue is: it doesn't matter. Using the relatively safe assumption that every dollar borrowed from Social Security is paid back: It won't be enough!!!
This is what I'm trying to get you, knarf, and all of the other mis-informed people on FR to understand: Stop whining about non-existent and trivial problems with Social Security.
Social Security has a real problem, but it's not the piddling crap that you complain about. Even if you were somehow able to magically wave them away, Social Security is still in trouble. By focusing on the non-existing and trivial problems, you are ignoring the elephant in the room:
There are no little "tweaks" that can fix it at this point. We passed that decades ago. We are now looking at a lot of pain for beneficiaries, taxpayers, or both. And the longer we wait to address it, the worse it will get.