If you then setup the portfolio again by borrowing $S_ t_1 $ at level $r$ you'll be able to realise a PnL at $t_2$ of $begingroup$ Assuming that you will be Doing the job for the bank, you will discover three various P&Ls dependant upon the functionality/ usage: You can https://www.youtube.com/watch?v=qMmsQ4kKgY4