Related for Gittin 50:8

(Translated from the Ehrfurt manuscript:) Any stipulation which has the action at the beginning, such a stipulation is invalid. How so? [If he, the brother-in-law whose brother died, said to her, his dead brother's wife,] "Now that I have released you from the levirate bond, with the understanding that my father is happy about it", even if his father doesn't want this—she is divorced [since he already released her with halitzah]. "Now that I have had sex with you, [be betrothed to me] with the understanding that my father is happy about it", even if his father doesn't want this—she is betrothed [since they already had sex, the stipulation is void]. Rabbi Shimon ben Yehudah says in the name of Rabbi Shimon: If the father wants it—she is betrothed, for she only had sex because of that original condition. Rabbi Shimon ben Elazar had a pneumonic: Any stipulation which is possible to do oneself or with an agent and he stipulated about it—his stipulation stands; but any stipulation that is impossible to do except with his body and he stipulated about it—his stipulation is void. How so? "Behold I divorce you with the understanding that my father is happy about it", if the father wants it—she is divorced; but if he doesn't want it—she is not divorced. "Behold I betroth you with the understanding that my father is happy about it", if the father wants it—she is betrothed; but if he doesn't want it—she is not betrothed.