Hotel Room, IQ Test

A test from Briquinex about the cost of a hotel room.
Julian, John and Christian go to a hotel and ask for one room. They are told that there is only one room available, which costs $ 300. Being late and they are tired to look somewhere else they accept. They pay each $100 and go to rest. Towards morning, while reviewing documents, the receptionist realizes that the room actually cost $ 250. So it sends the concierge to return $ 50.

On the way he thinks about how to split the $ 50 among the 3 equally. Not finding a satisfactory solution he takes $ 20 as a tip, and return each of them 10$.
Now if they were returned each 10$ , it means that each paid $ 90 each, 90 x 3 = $ 270 and with the 20$ taken by the concierge, results ​​$ 290. What happened to $ 10?