I think that the context makes it pretty clear that he's talking about borrowing and lending between friends. Shakespeare would indeed have been aware of it, as it was commonplace for young noblemen to run up ruinous debts from gambling and borrowing from each other and moneylenders (who were only too happy to lend to the heir to an estate). Families lost their wealth and estates because of it. "Neither a borrower nor a lender be" is extremely good advice in this context. And, frankly, remains so.

Also, I reckon people are generally too harsh on Polonius. Sure, he's not the sharpest tool in the shed, but in this scene he just seems like an old man who's worried about his son leaving home and trying to give him good advice. That he can't come up with anything genuine to himself is, I think, a little sad.

But I suspect most of us have endured our parents sending us off with good advice on our first time leaving home. And generally that advice seems to be similarly cliched ("Remember to eat properly...and make sure when you're going out, you know how to get home...and wrap up warm when it's cold out..."). I doubt I'll do much better when my time comes!