A user has ₦20,000 in their NGN wallet and wants to convert ₦15,000 of it to USD. Your quoted rate is ₦1,500 per USD ($10 gross). You charge a 1% FX margin: user receives $9.90, you keep $0.10. The lesson: you CANNOT put both sides of this in one journal entry, debits in NGN and credits in USD don't sum to anything meaningful. Each currency is its own balanced book, linked by FX Clearing accounts.