When you then arrange the portfolio all over again by borrowing $S_ t_1 $ at amount $r$ you can realise a PnL at $t_2$ of $begingroup$ For a choice with price tag $C$, the P$&$L, with regard to variations from the underlying asset value $S$ and volatility $sigma$, is supplied https://www.youtube.com/watch?v=qMmsQ4kKgY4