Kiddushin, Chapter Three, Mishnah Six
The first section of the mishnah teaches that a man may stipulate that his betrothal to a woman is contingent upon his performing for her a certain favor.
The second section of the mishnah discusses a man who makes his betrothal contingent upon his fathers approval.
1) If he says to a woman, Behold you are betrothed to me on condition that I speak to the government on your behalf, or That I work for you as a laborer, if he speaks to the government on her behalf or works for her as a laborer, she is betrothed; if not, she is not betrothed.
2) [If he says,] [Behold you are betrothed to me] on condition that [my] father consents, if his father consents, she is betrothed; if not, she is not betrothed.
a) If his father dies, she is betrothed; if the son dies, the father is instructed to say that he does not consent.
Section one: This mishnah is fairly straightforward. We should note that the favor is not the money for kiddushin itself. Rather it is a condition upon which the betrothal is contingent. In addition to the act which he must perform, according to halakhah he would also need to give her money to effect the kiddushin.
Section two: The first part of this section is straightforward. Slightly more complicated is the second part. If the father dies, he cannot consent to the betrothal. Nevertheless, the mishnah rules that she is betrothed. The assumption is that when the man said on condition that my father consents he really meant to say, on condition that my father does not disapprove. Since after his death the father cannot disapprove, the betrothal is valid. If the son dies, the mishnah finds a convenient way for the woman to avoid the need for yibbum (levirate marriage to her husbands brother). The court would instruct the father to state that he did not approve of the marriage and he would thereby annul the betrothal. If the betrothal was annulled, the woman would not be liable for yibbum.