Any time you then build the portfolio once again by borrowing $S_ t_1 $ at amount $r$ it is possible to realise a PnL at $t_2$ of WillWill 13344 bronze badges $endgroup$ 4 $begingroup$ Did you not say at first that $V$ is self-financing? In that scenario there's no cost https://www.youtube.com/watch?v=qMmsQ4kKgY4