i sell QQQ 201204C00303000 ( Sell call of 4th December 2020 expiry ) at 2020-11-23 for 100 quantity but it will close QQQ 201204C00303000 position at 2020-12-04 ( At the end of expiry day ) with 10000 quantity. If 100 quantity sell then it need to buy 100 quantity to close the trade, Why this type of difficulty comes during backtesting