Tosefet Bracha - Terumah 2

“They shall make an Aron of cedar wood, two and a half amot (amataim vachetzi) is its length” (25; 10)
The Talmud (Sanhedrin 29a) learns from here that anyone who adds really subtracts. Rashi explains that if you would remove the aleph from amataim (two amot) it would read mataim, meaning two hundred. By adding the aleph you really subtract from the total number and make it only two amot.
But Rashi’s explanation is difficult to understand. If the Torah would have only written mataim we wouldn’t know which measure it was using. It may have meant 200 amot, or 200 tefachim or 200 etzbaot (or any other measure). Whereas the word amataim speaks specifically about the length of two amot.
In the Torah Temima we gave a different explanation from one of the Rabbis of the Talmud there. When it says ‘anyone who adds, subtracts’ it refers not to the aleph, but to the vav of vachetz (and half). If it would have left out that letter the verse would have read ‘two amot is half its length’ which would imply a total length of four amot. By adding the vav the Torah teaches us that the length is only two amot, and therefore is subtracting. Even though in the Torah Temima we challenged this explanation, it nevertheless seems to be the most correct reading.

(editor’s note: in the Torah Temima he cites this in the name of the Rashash who attributes it anonymously. I also saw this explanation in the sefer Kol Eliyahu by the Vilna Gaon).