This question seems to be asking the Chakirah whether we look at the initial intention which was l'Shem Shabbos Kodesh and therefore not deductible from his allocation, or at the final result which was that the food purchased was not eventually used for Shabbos but for weekday consumption and is, therefore, deductible, becoming part of his annual budget allowance.

This could depend on the famous Machlokes as to whether the Hotza'os Shabbos v'Yom Tov u'Banav l'Talmud Torah is Davka or Lav Davka. According to the Shitah Mekubetzes b'Shem ha'Ritva, it is Lav Davka and no Mitzvah is deductible. If so, it is very Mistaber to say that everything that Shamai bought for Shabbos, even if not subsequently used for Shabbos, was not deductible as it was certainly purchased l'Shem Mitzvah.

However, according to the normative view as would appear to be the view of Tosfos (Beitzah 16a DH Kol) that only what can be derived from 'Chedvas' is included, namely Shabbos and Talmud Torah, it is arguable that only what is actually used on Shabbos is not deductible.

I think that the first Tzad of the Ritva is Muchrach. As to the reversed Tzad in the opinion of Tosfos, one can still argue the point. Indeed, perhaps we can bring a beautiful proof from Rashi that even according to this opinion none of it is deductible. In DH v'Ochel Es ha'Rishonah he writes, "It turns out that he eats this one so that the better one is eaten on Shabbos, therefore, the eating of the first one is li'Chvod Shabbos."

Rashi thus emphasizes that even the weekday eating is considered as being indirectly in honor of the Shabbos and the implication naturally follows that it is not deductible.

Kol tuv,

Joseph Pearlman