(Mishnah): If a man says 'I knew about vows, but I did not know about Hafarah', he can annul [when he finds out];

'I knew about Hafarah, but I did not know that this is a vow' - R. Meir says, he cannot annul (at all. There is no 'Yom Shom'o' when he first heard and knew that it is a vow). Chachamim say, he can annul (that very day).

(Gemara - R. Ze'ira): R. Meir's reason is, he seeks a pretext for her to vow (and it will not be annulled), and he can divorce her [without a Kesuvah, for one may say 'I do not want a wife who vows.'] If not [that he knew the Halachah, and merely seeks a pretext], he should have annulled initially! (Even if he did not know that it is a vow, there is no loss to annul it.)

(Mishnah): If one vowed not to benefit his son-in-law, and he wants to give his daughter money, he tells her 'these coins are given to you for a gift, as long as your husband has no Reshus over them. They are only for you to buy [food with them] and put it in your mouth.'

(Gemara - Beraisa): [He said 'your husband has no Reshus over them], and not you.'

(R. Mana): Who is the Tana [of the Beraisa, who says that he must say] 'and not you'? It is R. Meir, for R. Meir makes a slave's Yad (power of acquisition) like his master's Yad (the master acquires whatever his slave acquires, and also a woman's Yad like her husband's Yad);

Inference: It is because he said 'and not you.' If he did not say 'and not you', the woman acquires them, and her husband acquires.