I don't know for sure for forex trading, but I wouldn't expect it to be different from other kinds of trade.
There your buy offer contains the quantity as well as the maximum price for each item you are willing to offer. If someone offers you the same for less money, the trade should happen for this amount.
To be more exact, the price is determined in the following way:
- Assume the order book for a given "thing" (be it apples, dollars or Google shares) is emty.
- A wants to sell 4000 of it at a minimum price of 7500 €. As no one is here to buy, the order is added to the order book.
- B wants to buy 1000, but is not happy with the price and wants to pay only 7100 €. So another order is added to the order book, but on the other side.
- C wants to sell now 1500 at a price of 7000 €. As there is already a matching offer, 1000 of these can be sold to B at their price (7100 €). A sell order of the remaining quantitiy of 500 is placed at price 7000 €.
Now, various buy and sell orders can be placed, the ones which can be matched are executed, the remaining ones are placed into the order book.
The price is determined by the orders which are already contained in the order book.
I explain the way how it goes in a simplistic way, but that's about the way how it must be imagined.