בבא בתרא סג.
"בן לוי שמכר שדה לישראל"
If a לוי sells his field to a ישראל and adds a תנאי that the מעשר ראשון from this field should be given to him, the condition is binding. The Gemara concludes that this is not an issue of "אין אדם מקנה דבר שלא בא לעולם", because we view it as the לוי (מוכר) retaining a portion of the field for himself that represents a tenth of the produce of the entire field. Manyראשונים [מאירי, רשב"א] ask if we are saying that the לוי withheld a portion of the field, how may the לוקח use that produce as מעשר ראשון for his field? He doesn’t own this portion? תוספות is bothered why isn’t this a איסור of "כהן המסייע בבית הגרנות" i.e.; a כהן may not help out a ישראל in his wheat silo as an incentive to receive the תרומה, as this is a בזיון התרומה. Tosfos explains that this is different, because על פי דין the מעשר belongs to the לוי, so there is no issue of כהן המסייע and although he retained a piece of the קרקע, he gave the כח and permission to the ישראל to use his produce as מעשר ראשון on the whole field. This also explains the above question of why this מעשר may be used for the entire field. [אות רפ] ר' אלחנן elaborates and explains this idea as a similar halacha we have, regarding an עבד. If an עבד is accidentally killed by someone, there is a קנס that must be paid to the owner. The דין is, that one may keep his עבד but sell the rights to any future קנס that might be paid because of this עבד. So, he retains ownership but sells off one aspect of the ownership, receiving possible future קנס. Here, too, the לוי retains ownership of these פירות through שיורי משייר, so we avoid the issue of דבר שלא בא לעולם, but sells to the לוקח the right to use them (קורא שם) to separate מעשר ראשון for his own פירות.
The [הל' מעשרות פ"ז,כ'] רמב"ם paskens in our case that the תנאי is קיים (שיורי שייריה) and the לוי is entitled to the מעשר. The[יו"ד ס"א,כט] שו"ע paskens if a כהן sells a בהמה to a ישראל and makes a תנאי that the מתנות כהונה of this בהמה should go to him, the sale is valid, but the תנאי is בטל. The ט"ז asks, that theרמב"ם [ביכורים פ"ט,יא] agrees with this הלכה that בטל התנאי and the ישראל does not have to give the מתנות to this כהן, yet by us he paskens that the תנאי is קיים and the לוי gets the מעשר? The ש"ך answers that by the מתנות כהונה we can’t say he retained a portion for himself [שיורי שייריה], because he gave the entire בהמה to the לוקח. But by us, we are dealing with קרקע and קרקע בחזקת בעלה עומדת, so we can say his לשון of תנאי is that he retained a portion for himself. ר' חיים קניבסקי [דרך אמונה הל' מעשר פ"ו אות קפג] brings the מאירי and answers that in our case of the לוי, this תנאי is something that’s in place for years to come and anything could happen, like another לוי could grab the מעשר, and as a result, this makes it hard to enforce this תנאי, so he certainly means to be משייר. However, in the case of מתנות כהונה having to give the portion to the כהן happens close after the sale, so there was no intention of the מוכר to be משייר, so it remains a תנאי, which may not be done. (See רש"י חולין ד"ה ורמינהוfor the reason.)