The Mishnah (2:9) lists a number of objects from which one is prohibited from deriving benefit (issurei hanaah). The Mishnah teaches that if one attempted to perform kiddushin with one of these items it would not work. We shall try to understand why.
The Mishneh LeMelech (Issur Hanaah 5:1) uses the following case to probe why kiddushin cannot be performed with an issur hannah. He asks whether an issur hanaah could be used for kiddushin for a woman who was dangerously unwell. In that context that issur hanaah is permitted to her. He explains that the answer depends on how we understand our Mishnah. If we say that an issur hanaah normally would not work, because he needs to give her something that she can benefit from, in this case it should work because she is allowed. If however, the reason is because it has no monetary value, then even in this case, it is not considered as if he gave her anything. Finally, if we say that it would not work since would be deriving benefit for an issur hanaah through the process of kiddushin, then it would similarly not work in this case.
The Mishneh Lemelech (s.v. ve'raiti) proves that Rashi maintains the first understanding based on his explanation of the end of the Mishnah. The Mishnah explains that if the man sold the issur hanaah and then used the money for kiddushin, it would work. Rashi explains that this is because the money from the sale does not adopt the issur hanaah and has monetary value. Rashi however adds that even though he is still not allowed to derived benefit from it, she can. The Mishneh Lemelech understands that Rashi's position is that it has not monetary value for him since it is rabbinically prohibited for him to derive any benefit. Consequently, for him it is not considered as if his giving her anything. Nevertheless, the kiddushin would work since she received something that she can benefit from. It follows then that Rashi would maintain that in the case where she is dangerously ill, since she could derive benefit from the issur hanaah, she would be mekudeshet. Furthermore, we can prove from Rashi that the reason why we he cannot use an issur hanaah for kiddushin cannot be because he derives benefits through the process, otherwise even the money that was derived from the sale of the issur hanaah would similarly not be able to be used.
The Mishneh LeMelech explains that the Ran also maintains this position. He explains that the money from sale can be used, even though he cannot derive benefit, because she is allowed to and it came into her possession because of him. He proves this from the case of kiddushin with gezel -- a robbed object. Kiddushin would work with gezel according to the opinion that in a case of gezeila (robbery) the victim gives up hope of retrieving it (yi'ush). The object is not considered the property of the robber because yi'ush alone is not enough to change the ownership. It requires shinui reshut, a change in possession also. Nevertheless, shinui reshut occurs when he gives it to her and it is then an object she can benefit from. Consequently, since it the came into her possession because of him, kiddushin would work.
The Mishneh Lemelech (s.v. shuv) however notes that the Ritvah disagrees. He argues that kiddushin would not work with an issur hannah for this woman who is dangerously ill. He explains that for kiddushin to work he needs to give her something of monetary value. Even though it has value to her, since it is an issur hanaah, he would not be able to sell it to her. Consequently, it has no monetary value. (If however, he say he was being makesh with the benefit she would derive from it rather than the object itself, it would indeed work.) We find therefore that the Ritva maintains the second understanding.
What would the Ritva do with the proof of the Ran above? The Mishneh Lemelech explains there is a difference between the two cases. In the case of an issur hanaah, even after he gives it to her, it remains forbidden to him. In the case of gezel however, due to the shinui reshut, it came into her possession through him. The issue in the case of gezel was that due to the lack of shinui reshut it did not belong to him. Once she however acquired it, he did too.
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