Why Do You Eat Terumah

 וְכֹהֵ֗ן כִּֽי־יִקְנֶ֥ה נֶ֙פֶשׁ֙ קִנְיַ֣ן כַּסְפּ֔וֹ ה֖וּא יֹ֣אכַל בּ֑וֹ.  Rashi cites ואשת כהן אוכלת בתרומה מן המקרא הזה שאף היא קניין כספו.  Rashi Ketubot (58a) adds וכי היכי דקידושי ביאה מאכילין בארוסה דאיתקיש הוויות להדדי.  [I'm not sure if Rashi here on Chumash that omits this point disagrees or is just citing the basic din mentioned in the Gemorah.]  From the fact that Rashi needs the hekesh to know how if the kinyan kiddushin not done through money the kohan can still feed his wife teruma, it seems he understands קנין כספו is literally a kinyan done through money.  If so, how do we know that a slave acquired through chazakah or shtar can eat terumah?  Rav Elchonon (Kovetz Haoros) says when it comes to the slave the words קנין כספו are referring to the type of kinyan one has in the slave; that is the monetary kinyan.  However, one doesn't have a monetary kinyan through kiddushin, so קנין כספו can't refer to the type of kinyan, it must refer to actually acquiring through money and hence Rashi needs the hekesh for the other kinyanim.  It comes out according to Rav Elchonon, that the slave eats terumah because of the קנין ממון his master has in him but a wife eats because of the קנין איסור acquired through money.  The difficulty is if we assume that קנין כספו regarding the slave refers to the type of kinyan, then how does the Gemorah know that it also refers to the literal קנין כספו of kiddushin?

Even though one doesn't acquire any monetary gain through kiddushin, the Achronim bring many proofs [and it is the simple read of the Gemorah in the beginning of Kiddushin,] that the act of kiddushin, of acquiring the women so to speak, is a monetary kinayan just like any other kinyan when one acquires ownership the ownership rights is a קנין ממון in the object.  The Maseas Moshe on Kiddushin suggests that is all true when the kiddushin is done through money but it its done through shtar or beah then the kinyan will be defined as a eishus type of kinyan.  He says, if we follow this approach, then we can understand the Rashi.  No matter how a slave is acquired, one is acquiring a קנין ממון is the slave and can feed it terumah and the same is true when doing kiddushin though money.  When doing kiddushin with another method, then the kinyan isn't defined as a קנין ממון and Rashi needs to invoke a hekesh in which we learn out that when does the other kinyaninm he ends up with a קנין ממון in his ארוסה as well.  If we take this approach it comes out the reason a slave and ארוסה eat terumah is because of the קניו ממון that one has in them.  However, the chiddush of the Maseas Moshe it tremendous and one can definitely debate its truth.     

The אבני מלואים in Teshuva 17 learns up in Tosfos Yebamot (66a) that its the קנין איסור one has in his slaves that allows him to feed them terumah.  In his view קנין כספו means literally acquired with money.  According to his approach, how do we answer the question of Rav Elchonon, why do we need a hekesh for the other kinyanim of kiddushin, but not for the other kinyanim of the slave?   I was going to suggest an answer but I would rather leave it as a question and maybe someone can suggest an answer.