בבא בתרא קלז:
"מתנה ע"מ להחזיר"
The Gemara [סוכה מא:] learns from the pasuk, ולקחתם "לכם" ביום הראשון, that the ד' מינים must belong to you on the first day of סוכות. This excludes שאול and גזול. ר"ג ,ר' יהושע, ראב"ע, ור"ע were on a boat and ר"ג was יוצא with his esrog and then gave it to ר' יהושע as a מתנה ע"מ להחזיר so he may use it. The Gemara explains that although it was given on the condition it be returned, it is still considered לכם.
Meforshim debate how to view a מתנה ע"מ להחזיר. The [סוכה פ"ג, ס' ל] רא"ש learns that the only way the item being given may be considered "לכם", is if we view a מתנה ע"מ להחזיר as a "מתנה גמורה" with a rider attached to it that stipulates it must be returned. If it is just given for only a specified זמן, then the receiver would be considered a שואל and it wouldn’t be לכם. Consequently, since it is a מתנה גמורה, the מקבל must be מקנה it back when the time is up. He brings a proof from [מו:] ר' זירא that says one should not be מקנה his esrog to a קטן as he can’t be מקנה it back. Why not give it to him as a מעמ"ל? We see that this works as a gift לעולם and the קטן would need to be מקנה it back, which he can’t do. The קצות [רמ"א, ד'] says "לולא דברי הרא"ש" he would say that even if originally given only for a זמן, it would not be considered שאולה and it would be לכם. According to the קצות, after the predetermined time, the item reverts back to the נותן automatically, while the רא"ש holds the מקבל must actually be מקנה it back.
The [חו"מ רמ"א,ו'] רמ"א says the אומדנא is that when a giver stipulates that it be returned to him, it is because he still wants to use it. Therefore, it is understood that he return an esrog right away so the נותן may use it for the rest of יו"ט. If, however, the נותן just says ע"מ שתחזיריהו but doesn’t say "לי", the מקבל may return it whenever he wants. Also, if one makes a נדר to give ראובן a מתנה, he may not give it ע"מ להחזיר. The סמ"ע explains since he specified ראובן, it’s משמע he is נודר על דעת ראובן, which is not ע"מ להחזיר. But if he just made a נדר to give a gift, he may give it to anyone with a תנאי ע"מ להחזיר and he has fulfilled his vow.
The Gemara says [קידושין ו:] that מעמ"ל works by: מכר,נתינת תרומה,פדיון הבן. ר' אלחנן wonders how מעמ"ל works for פדיון הבן. In essence פדיון הבן is a פריעת חוב and certainly paying off a debt ע"מ להחזיר the money afterwards can’t work. Just like one can’t give צדקה to a עני and stipulate ע"מ להחזיר. However, the [יו"ד רנ"ח,יב'] ש"ך brings the ר"י that one may be יוצא צדקה by giving עמ"ל!
The רא"ש brings ר' ישעיה who explains the difference between a שואל and a מקבל of a מעמ"ל, is that a שואל is חייב if an אונס happens to the item, whereas the מקבל is פטור because he has received it as a gift (with a stipulation that if it’s still בעין he must return it). The רא"ש argues, but the שו"ע[חו"מ רמ"א,ח']] paskens that the receiver of a מתנה ע"מ להחזיר is פטור באונסין.