After the Trade
John’s screen has recorded that he has used EUR 20 million of his daylight limit. It also shows a loss of USD 2,000 on the trade despite prices remaining constant. This is because of the spread between the bid and offer prices.
John bought EUR 20 million at 1.3798. John can only square his position by hitting the bid, that means selling the base currency, EUR, to the market maker and purchasing the variable currency, USD, at a rate of 1.3797. This creates a loss of USD 2,000 (= EUR 20,000,000 x (1.3797-1.3798)).
Change in Spot Price
The spot rate is continuously changing causing John’s profit and loss to change with it. If the EUR/USD price falls he will have to square the position at the lower price showing a bigger loss than just that with the spread. Conversely, if the price rises his loss will reduce and eventually create a trading profit once the bid rate goes above his breakeven level of 1.3798.

Calculating Profit & Loss

To understand how the profit or loss is calculated, let's look at an example. Alice, an FX trader, takes a long position (as a price taker)  in the Thai baht (THB) at these prices: