לע"נ סבי ר' שמואל פנחס בן ר' יעקב צבי 

The opinion of Tosfos [Sukka 35] is that one cannot be yotzei with an Esrog of Tevel because the Kohen is a partner in the Esrog and it thus lacks the requirement of לכם. The Maharit Algazi asks on this Tosfos from the machlokes haposkim as to whether one is פטור from הפרשת תרומות and מעשרות if he buys the fruits after מירוח [the smoothing of the pile - see Bava Metzia 88a with Tosfos ד"ה תבואות]. 

BUT WAIT!!! How can he sell the Tevel in the first place?? If it lacks the quality of "לכם" - as we see with regard to the halacha of an Esrog of Tevel, then it should also lack the quality of "לכם" when it comes to selling it and one should not be allowed to sell it??  

Answered Maran Rosh Hayeshiva HaRav Hutner ztz"l: The Ramban [in Sukka] asked on Tosfos - Why should Tevel be considered the property of the Kohen? The owner can separate his תרומות and מעשרות from other produce and keep the entire Esrog for himself??! 

The key to solving these issues is a principal of the Mikdash Dovid [of HaRav Dovid Rappaport ztz"l Rosh Yeshiva in Baranovitch] in Hilchos Terumos [אות ה]: The primary terumah is is הפרשה מיניה וביה - separating from the fruit itself and not from elsewhere. If one so chooses, he may then prefer option "b" - and separate from elsewhere. But option "b" requires a NEW קביעות, a new establishment of teruma and not the primary קביעות which is from the very produce one is rectifying. עיין שם דבריו העמוקים!!!

Now Tosfos makes a lot of sense!! Since at the time one takes the Esrog it stands to be separated from and given in part to the Kohen, it cannot be considered "לכם". The fact that he can make the Esrog his own by separating Terumah from elsewhere doesn't make it his. In order to be considered "לכם" it must be his מיניה וביה - from the Esrog itself. Option "b" is a new קביעות and doesn't impact the halacha of "לכם", since on the basic, primary level it belongs at this time to the Kohen. 

However when it comes to the concept of "בעלות" - ownership, Tosfos agrees with the Ramban that it suffices that one can remove the Kohen from his share in the Esrog and make it his by separating from elsewhere, in order that it should be considered "לכם". [The standard for לכם with respect to קיום המצוה is more strict than it is for דין בעלות].   

  [משנת תשל"ד]