Why can I buy $10,000 worth of L$ on the LindeX but only sell $9,650 worth?

If I buy $10,000 worth of L$ at L$281/1 I get L$2,810,000. If I sell $10,000 worth of L$ at L$270/1 I need L$2,700,000 but after the 3.5% commission I only get $9,650. I don't lose money because I still have L$110,000 that is worth $393.15 at the L$270/1 exchange rate or a profit of $43.15. However if I can only sell or buy $10,000 worth of L$ per month and always buy and sell that amount I wind up with L$110,000 every month times the number of months I do that. I can't actually take profits unless I stop trading. Why don't I get to sell the $10,000 worth of L$ after the 3.5% commision instead of before it?