Saday writes an ejective s and continues the Semitic consonant behind the Hebrew tzaddi, the Syriac sadhe and the Arabic sad. The correspondence is direct and uncontested.
What is unusual is that Ge'ez has a second letter of the same class. Sappa, the letter that follows this one in the order, wrote another emphatic sibilant, and Hebrew and Syriac have no counterpart to it because their branch merged the two before their alphabets were fixed. Arabic keeps both, writing the second one with dad. So on this pair Ge'ez agrees with Arabic against the Northwest Semitic scripts, which is a recurring result across the fidel and a reason the atlas records Arabic cognates on several of these letters rather than only Hebrew ones.
In the received Ethiopian reading tradition the two have fallen together and are sounded alike, so the distinction survives only in spelling. A scribe cannot hear which letter a word takes and has to know it, which is the same situation as the two h letters and the two plain s letters. Four separate mergers give the fidel more signs than a modern reader has sounds, and every one of them is preserved because the classical text is fixed.
The four mergers are worth counting, because between them they are the reason a beginner finds the fidel harder than its size suggests. Twenty-six letters is a small alphabet. Twenty-six letters of which eight fall into four pairs that sound alike is a spelling system, and a spelling system has to be learned word by word in the way English spelling does.