If you then create the portfolio once again by borrowing $S_ t_1 $ at level $r$ you are able to realise a PnL at $t_2$ of $begingroup$ For an alternative with price $C$, the P$&$L, with respect to improvements from the underlying asset selling price $S$ and volatility $sigma$, is https://www.youtube.com/watch?v=qMmsQ4kKgY4