Rav Kornfeld - Shalom Rav.
Regarding Maalin BaKodesh V'ein Moridin (Megila 26)
The solution of the RAN as quoted seems difficult to accept in practice: assume the people in charge (the Gabaim) did in fact buy an object of lower sanctity - must the seller, who perhaps made the object to order of the Gabaim - lose out? Even if the sale was bone-fide on the part of the seller, who was only a merchant - must he return the money and be stuck with the goods?
Perhaps the Gabaim have to replace the money to the public coffers?
In that case it may be said that this is new money and we have indeed done "Moridin" with the old; should we then say that this "Ein Moridin" is a "Lav Shenitak LeAsei" an can be "corrected" by the new monies?
I would think that a much simpler and direct answer is:
Lechathila - first choice - is Maalin BaKodesh; however, it may well be that the Tsibur has no need of objects of higher sanctity at present, and does have need of objects of equal sanctity; in this case we apply may forgo "Maalin BaKodesh", and purchase goods of sanctity equal to that of the objects which were sold.
However, when there is a real need for objects of the lower sanctity, but there is no need for objects of equal or higher sanctity - in this case "Ein Moridin" applies and ther money may NOT be used to but the neccesary objects.
Both rules: "Maalin BaKodesh" and "Ein Moridin" - have different spheres of applicability and neither is superfluous.
Yeshayahu HaKohen Hollander
Your points are correct (however, it appears that the rule of "Ma'alin b'Kodesh" is not applied specifically to the money received for the sale of the Sefarim, but rather even if that money was exchanged for other money, or it was all spent, or irrevocably lost (it fell into the sea), there is still an obligation upon the city treasurers to buy an item of greater Kedushah with the value of the money that was received from the sale of the Sefarim. If so, even if there is no need for items of greater Kedushah, and they bought items of lower or equal Kedushah with that money, the obligation still remains upon them to buy something of greater Kedushah.)
In fact, after we wrote the explanation of the RAN, we discussed the Sugya with our friend R' Mordechai Rabin, and we realized that the Ran is saying something other than what we initially understood. The Ran's intention seems to that only if a Sefer Torah is sold, since it is impossible to buy something of greater Kedushah, the sale of the Sefer Torah is invalidated (and not the sale of the items bought with the money received for the Sefer Torah).
To answer our original question on the words of Rashi, R' Mordechai Rabin answered based on the other approach that the Ran writes, when he suggests that a Sefer Torah -- since there is no higher Kedushah and the only possibility is to buy something of equal Kedushah, the Chachamim permitted buying something of lower Kedushah, since the requirement to be Ma'alin ba'Kodesh anyway will not be fulfilled. According to this, perhaps this is what the addition of "Ein Moridin" teaches -- even where it is impossible to buy something of a greater Kedushah (such as when selling a Sefer Torah), nevertheless one may not buy something of a lower Kedushah. According to this approach, it is understood why Rashi mentions this Derashah in the Seifa of the Mishnah which says, "One may not sell a Sefer Torah and buy [with the money] Sefarim...."
Mordecai Kornfeld