Kiddushin, Chapter Two, Mishnah 6. This mishnah deals with a kiddushin that lacked a sufficient amount, and with the question of how to view gifts sent to the woman after such a kiddushin.
"HaMekadesh shtei nashim b'shaveh perutah, o ishah achas b'pachos mi'shaveh perutah":
"HaMekadesh shtei nashim b'shaveh perutah" - a man who betrothed two women with a single object worth only one perutah. This is not enough, since each and every woman requires a perutah's worth for herself.
"O ishah achas b'pachos mi'shaveh perutah" - he betrothed one woman, but with less than a perutah's worth. In both cases the kiddushin is not valid.
"Af al pi sheshalach sivlonos l'achar mikan - einah mekudeshes":
Sivlonos are gifts that a man customarily sends to a woman to whom he has already given kiddushin. The mishnah teaches that even if he sent these gifts after an act of kiddushin that was not valid, the woman is not betrothed through them. We do not reinterpret the gifts and say: the man realized that the kiddushin he performed was insufficient, and therefore he is using these gifts for the purpose of kiddushin. We do not say this.
"Shemachmas kiddushin harishonim shalach":
The reason is this: we attribute the sending of the gifts to his expectation that the first kiddushin was valid. Even though in fact it was not valid, he believed that it was, and we do not assume that he understood that it was defective and came to complete it by means of the gifts.
"V'chein katan shekiddesh":
The same law applies to a minor - someone who has not yet reached the age of bar mitzvah - who betrothed a woman, and whose kiddushin was not valid at that time. If, after he grew up, he sent her sivlonos, we do not assume that he understood that the kiddushin of his childhood was void and that he is now giving the gifts for the purpose of kiddushin. Here too we attribute the gifts to the kiddushin that he thought was valid but in reality was not, and therefore there is no kiddushin here.
In summary: In this mishnah we learned that kiddushin performed with less than a perutah's worth for each woman does not take effect, and that gifts sent afterward do not serve to render the kiddushin valid. The reason is that we attribute the sending of the sivlonos to the first kiddushin, which the sender mistakenly believed to be valid - and the same applies to a minor who betrothed a woman and sent sivlonos after he grew up.