Halakhah su Deuteronomio 25:5

“On the condition that my father will not protest,” if he hears and protests she is not betrothed. If he does not protest or dies behold she is betrothed (see close to here end of paragraph 10), if the son dies and afterward the father hears,¹¹⁵of the proposed betrothal. we instruct the father that he should say “I do not approve,” in order that there will be no betrothal and that she will not be subjected to the position of Yibbum.¹¹⁶This refers to the marriage between a widow whose husband died without offspring (the yevamah) and the brother of the deceased (the yavam or levir) which is prescribed in Deuteronomy 25:5-6. There are those who say that one who says “on condition that he¹¹⁷i.e., the father. approves” has the same ruling as one who says that he will not protest. There is one that says that the ruling is the same as one who says: “on condition that he be silent.”

The commandment of eating the remainders of the meal-offering: That the priests were commanded to eat the remainders of the meal-offerings - meaning to say after they separated from it that which they would offer on the altar, they would eat all the rest, as it is stated (Leviticus 6:9), "What is left of it shall be eaten by Aharon and his sons; as matsahs shall it be eaten, etc." And the language of Sifra, Tzav 2:9 is "'As matsahs shall it be eaten' is a commadmeent; 'her levirate husband shall have sexual relations with her' (Deuteronomy 25:5) is a commandment" - meaning to say that both of them are positive commandments, not optional.

Someone whose brother by his father died and did not leave descendants, it is a commandment to perform levirate marriage with his wife, whether she is his wife from marriage or his wife from betrothal, as it is written: "When brothers dwell together and one of them dies and has no son, her yavam shall go in unto her" (Devarim 25:5)