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Help me understand Forex in Interactive Brokers
You're confusing open positions and account balance. Your position in GBP is 1000, that's what you've bought. You then used some of it to buy something else, but to the broker you still have an open position of 1000 GBP. They will only close it when you give them the 1000GBP back. What you do with it until then is none of their business. Your account balance (available funds) in GBP is 10.