I just saw some loan/bank ad that read something around the line "when the budget goes negative, the minus becomes plus" ("Când bugetul dă cu minus, minusul se face plus")

NO, this isn't a feature, it's a bug and it's called "underflow"

I don't understand how this made its way into an advert.

For understanding how these low interest loans work, I would suggest you to go to the following website https://loans.no/forbrukslan/. There is no better website for this purpose.

When you have no money it's bad anyway,

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