I think Rumpelstiltskin always knew. If he was going to make “a curse to end all curses” ofcourse he would have some sort of loophole, some sort of advantage. The please card is an example because its going to come in handy later on. You never break a deal with Rumpelstiltskin.
Its just Rumpelstiltskin’s style to not use please for eveything. Thats just too easy and will make him look like a coward. Rumpel knows when hes lost and when to give in. That was one of those moments. Rumpel has been alive for a long time, so he has power and experience; and Regina only has power.