When you then setup the portfolio again by borrowing $S_ t_1 $ at amount $r$ it is possible to realise a PnL at $t_2$ of $begingroup$ For an alternative with price $C$, the P$&$L, with respect to improvements of your underlying asset price $S$ and volatility $sigma$, is provided by https://pnl32186.bloguerosa.com/33066845/how-pnl-can-save-you-time-stress-and-money