There should be something wrong with this question. The maintenance margin in question should mean that the excess of three futures contracts is $4,000, otherwise the maintenance margin will be greater than the initial margin (generally called the initial margin) and the maintenance margin should be less than the initial margin.

The deposit that the customer should pay is the initial deposit multiplied by the contract quantity, that is, 2700*3=8 100 USD. If the next day, the pound is discounted by 300 points (each point here is actually 0.000 1), that is to say, the exchange rate of each pound against the US dollar has dropped by 0.03 US dollars. Because the customer bought more pounds, the discount of pounds means that the customer is losing money, so the customer's profit and loss is 62500*0.03= 1875. Since the initial margin of the three contracts is 8 100 minus the loss of $5,625, the actual margin balance is only $2,475, which is lower than the requirement of maintaining the margin of $4,000, so it is necessary to make up the margin, that is, to make up the current margin balance to the initial margin amount, so the margin of $5,625 is made up.