To give an actual answer, Monopoly in Catan allows the player who played the card to name a resource, and every other player must give all of that resource to the player who played the card. I imagine the OP negotiated using a specific resource, got what they wanted, then monopolied the traded cards back.
Ye, I think his friends just regarded it as a dick move lol. Personally, I always enjoyed using the card later in the game when people start getting 2x resources every other roll.
My girlfriend's dad did that trick once. He asked if anyone was willing to trade stone, and once everyone piped up he played the monopoly card. She was so upset.