Your problem lies in confusing firm trading with margin trading and the criteria for calculating profit and loss. The formula you use is the calculation formula of margin trading, but you look at these two transactions from the perspective of firm offer.
First of all, come to a conclusion: if it is a firm offer, you have to buy dollars with the Japanese yen in your hand, and then change the dollars into Japanese yen. If these two transactions are denominated in dollars, you have nothing to lose. That is, the currency (yen) in your hand is still worth 100000 dollars. But if you calculate the profit and loss in yen, it is a loss of 50%. Because the hundred thousand dollars in your hand can only be exchanged for five million yen instead of the original ten million yen.
If it is a foreign exchange margin transaction, your net profit and loss is100000 USD. Because the process of margin trading is not that you buy and sell with the money in your hand, but that you borrow money from the developer to do the transaction. This is the biggest difference. It also brings you confusion.
To do margin trading, you must first have a certain margin yourself. If you assume that there is no lever, then the lever is 1. First, you put your100,000 dollars in the dealer as collateral. Borrow Japanese yen from the dealer to buy100000 USD. Because the exchange rate at that time was 100, then you owe the banker one hundred thousand * 100= ten million yen. When the price drops by 50, you close your position. At this time, you should use the100000 dollars in your hand to buy Japanese yen and return it to the banker. Because the price has fallen, I can only change my $100,000 into five million yen. You paid back 5 million yen to the market maker, and you still owe 5 million yen. This debt will be repaid with your savings. According to the calculation of USD/JPY =50 yuan, 5 million JPY equals 100,000 USD. So your savings are gone.
This shows that your margin did not really participate in the transaction, but only borrowed money from the brokerage firm to trade. Margin is only a threshold for trading, which is used in the final profit and loss calculation.
View the area. A firm offer is how much you lose by trading with your own money (of course, it also depends on what currency you use, and China is of course denominated in RMB). Margin trading and gambling are the difference, and the calculation of profit and loss has nothing to do with the amount of margin.