What is meant by the impermissibility and invalidity of donating a undivided share (musha‘) according to some scholars? Does it refer to an entire residential apartment or to an undefined share thereof?

A specific apartment unit does not fall under the meaning of "mushaa'" (undivided common property). Rather, "mushaa'" refers to an unapportioned share of it with undefined boundaries. The fact that the share is specified in quantity, such as a quarter, a third, or a half, does not remove it from being mushaa' unless the apartment is divided and the share's boundaries are defined. Scholars have differed regarding the gifting of mushaa' that can be divided: the majority permitted it, while the Hanafis prohibited it. As for mushaa' that cannot be divided, the Hanafis agreed with the majority on the permissibility of gifting it. The state of commonality (shuyu') that prevents gifting, according to the Hanafis, is considered at the time of taking possession of the gift, not at the time of concluding the contract.