You could have said that the first time, would have saved a lot of time and prevented me from a 101% wrong guess 

What you're generating there is an invoice, it just like those machines that don't give change back....
Depending on what the policy of the exchange you use is they might credit you with the full sum the guy deposited on it or they might hold the extra for themselves.
Far better and a bit easier (if you still want to have your money on an exchange) with no time pressure, go to your exchange dashboard and get your deposit address. Talk to the guy, find out how much 100$ or 150$ in bitcoin is and ask him to deposit the 0.0xx BTC on that address.

Again, we don't have a clue what exchange your using but at least the biggest part of the mess has been cleared out.