I've been doing a deep dive into how instant exchanges work. Does an instant exchange function like a travel website that finds airfare from different airlines?

Is an exchange amount guaranteed? For example, I put in an exchange of 2 Coin As for an equivalent of 4 Coin Bs at the set price. What happens if there is not enough of Coin B available at the set price? I've already sent my 2 Coin As to the instant exchange's wallet. Is there a possibility of not all of the amount of the agreed upon exchange to be executed? If it is, what happens to my Coin A that I sent that is leftover?