You do own the stock, and when you sell it you no longer own it - the person you sold it to does. You are obliged/contracted to "return the/a stock" but there i

You do own the stock, and when you sell it you no longer own it - the person you sold it to does. You are obliged/contracted to "return the/a stock" but there is no requirement that it be the same stock, as the agreement is wrt stock as a commodity. Furthermore, that agreement doesn't in any way apply/affect to the stock you sold (or the person you sold it to), the obligations to return is yours alone.