After you then build the portfolio yet again by borrowing $S_ t_1 $ at fee $r$ you'll be able to realise a PnL at $t_2$ of $begingroup$ For an alternative with rate $C$, the P$&$L, with respect to modifications with the underlying asset value $S$ and volatility $sigma$, is given https://www.youtube.com/watch?v=qMmsQ4kKgY4