Tippy walks into a bar. The waitress pours him a pint of beer and he pays with all the money he has on him. He says to the waitress. "Sorry I can't tip you today, but you know what, I have this lottery ticket and if I win, I split it with you." The waitress says, No problem, Tippy." Tippy makes a copy of the ticket for the waitress, writing on it "If I win, half of this is yours." The next day Tippy receives a call from the waitress who tells him that he has won $1 million and that she wants her half. Tippy refuses.

is the gratuitous promise under seal?