Yup, the market interface will try to match whatever you tried to buy, but sometimes other people will buy out the available minerals before you can click OK. So in this case the market interface will create a buy offer, from you, with the price that you had showing in the box, and other players can see it and sell the minerals to you, if your price is the best.
You can check your buy offers from the Wallet, and right-click to cancel them if you want. You should get your money back, minus taxes and fees of course.
You can also do this on purpose; right-click on some minerals on the market but when the box comes up, specify your own (lower) price. The server will create a buy offer from you, at the lower price that you wanted, and hopefully in a couple days the prices go down and some player will sell his minerals to your buy offer.
It’s basically, pay high price and get it now, or offer lower price and wait a bit until it’s fulfilled.
EDIT: The escrow wallet holds the money that you have to pay when the other players finally click and accept your buy offer; they get paid with that money in the escrow and you get the minerals in your station hangar.