For example, someone set ask BTC/USDT limit offer to sell 1 BTC for 19000 USDT. In same time i setup limit order to buy 1 BTC, for example, by 19100 USDT..

In this case I got 1 BTC and i'll pay for 19100 USDT. The seller whose order I paid will receive 19000 USDT.
Where is 19100 -19000 = 100 USDT ? This is exchange profit or I wrong and exchange will not allow you to place such order?