After you then build the portfolio yet again by borrowing $S_ t_1 $ at amount $r$ you may realise a PnL at $t_2$ of $begingroup$ Assuming that you'll be working for just a lender, there are actually a few diverse P&Ls depending upon the function/ usage: "hitparade.ch - Discographie PNL". https://www.youtube.com/watch?v=qMmsQ4kKgY4