You didn't go below 0. At the end of all of this, you weren't at -$20 or something, you were at $0. The card isn't saying that its net effect will never be worse than +$0. It means what it says, you can't end up below $0.
You started with $17 and 18 cards in hand, then the 1st Souk gave you $7, then happily took away $17 (the played Souk wasn't in your hand anymore), leaving you at $7. The next Souk again gave you $7 (bringing you to $14), then it would have subtracted $16 considering had size, but only subtracted $14, to bring you to exactly $0.
