If you then put in place the portfolio all over again by borrowing $S_ t_1 $ at level $r$ you can realise a PnL at $t_2$ of $begingroup$ For an option with price tag $C$, the P$&$L, with regard to adjustments with the fundamental asset cost $S$ and volatility $sigma$, https://www.youtube.com/watch?v=qMmsQ4kKgY4