The broker carries out the order at the price set by the trader (or a set deviation). If it cannot get the price, no trade is made. If the order cannot be filled, the trader sees a “re-quote message” in which the trader can accept a new price. One advantage of FX swap is that you will enter the market at a fixed price.
The broker carries the trader’s order at the current price on the market when the order is processed (as opposed to when the order is received). The price may be either higher / lower than what the trader sees. The advantage is that the trader will get into the market when they place an order.