Theoretically, the exchange only earns from such trades through trading fees

I don't know if I understood your question well, but an ASK (sell limit order) can never be lower than a bid (buy limit order). If your buy limit order happens to be higher than the sell limit order, your sell order gets executed at market price as you set it up. That means you will buy the BTC at 19,000. Sell takes 19,000. You remain with 100 in your account minus the trading fees.