Mathematics

A Jewish salesman came to another town, but because he has not finished all his businesses there, he stopped for the night at an inn. Unfortunately, all the beds were taken. Fortunately, there was another Jew who was occupying one of the rooms, and, after agreeing to split the cost of the night, he agreed to share a bed with our salesman. Unfortunately, this Jew turned out to be very talkative: "Moniek arranged to buy out Rappaport's bill of exchange. Lipszyc who wants to ruin Rappaport buys his bills at the rate of 90. If I buy out 5,000 of these bills of exchange at the rate of 70, even if I have to give Moniek 3% commission, I will still get 850 zlotys!"
Our salesman said: "Stop it! You've made a mistake!"
The other Jew answered: "How could I have made a mistake?! The 90% paid by Lipszyc is 4,500 zlotys. I buy the bill of exchange for 3,500 zlotys. So even if I give Moniek 150 zlotys, I'm still left with 850 zlotys?!"
"You've made a mistake again!"
"How come? After all, 4,500 minus 3,500 is 1,000. Subtract Monk's 150 and I'm left with 850 zlotys! So what am I wrong about?!"
"You're scratching my butt!"