We are sorry you feel this way. Please be assured that we are definitely not scammers. We are required by our regulator, the FCA, to treat our customers fairly. This is a key obligation placed on us. An administrative error was made with the way the TRYJPY swap had been put in to the system, meaning you were getting a much larger swap from us than you should have. It therefore meant we were receiving a much smaller swap from our liquidity providers than we were paying you!
As a goodwill gesture, we therefore even allowed you to keep an extra EUR 120 more than you were entitled to, on your closed positions. We did not at any point take any money from you that your were legitimately entitled to. In fact, you are still receiving a positive swap on your positions. However this swap is the correct amount, which is lower than what you had been previously receiving.
Errors can be made by anyone, and we are within our rights to correct them as we become aware of them. In keeping with our obligation to treat customers fairly, had we taken extra money from you, to which we were not entitled, we would have been required to pay this back to you. So this issue works both ways.
One final point to note, you mentioned that if we make a mistake then it is our problem to deal with. This is not true. If your bank accidentally deposited $1 million in to your bank account by mistake, it would not give you the right to keep the money and spend it how you like, and then say it is the bank's problem. The bank would take back every dollar to which you are not entitled, as you have no right over this money. This is exactly what happened in this situation with the swaps.