halacha - Shamai HaZakein Buying for Shabbos All Week

The Gemara (Beitzah 16a) talks about Shamai HaZakein, who would buy meat every day, setting aside the current day's meat for Shabbos when superior to the previous day in quality, then he would eat the previous day's portion which is no longer set aside for Shabbos.

Once you've set aside meat for Shabbos, why isn't it huchshar (set aside) for Shabbos, and thus using it during the week would be an issue of ma'alin bakodesh v'ein moridin? (Related: Are you allowed to wear old Shabbos clothes during the week?.)

Second, his lashon was זו לשבת. Isn't that a lashon of a neder, in which case, even if it's not a ma'alin bakodesh issue, shouldn't it be an issue of violating a neder?

Answer

Even if Shamai made a Neder it was a Neder betaus i.e he mistakenly regarded this food as the best quality available but then he realized there was better

This is a Gemara in nedarim 65b where the definition of a mistaken neder is described and does not need hatoro (absolving)

מתיב רבי אבא קונם שאיני נושא לפלונית כעורה והרי היא נאה שחורה והרי היא לבנה קצרה והרי היא ארוכה מותר בה לא מפני שכעורה ונעשת נאה שחורה ונעשת לבנה קצרה ונעשת ארוכה אלא שהנדר טעות I.e If he thought she was ugly and made a neder not to marry her but was mistaken because she was actually beautiful his Neder is automatically batel (annulled)

So to here he thought this was the best food but it turns out there was better so he had inadvertently set aside the wrong food.

The source of Maalin bakodesh is with regards to kodshim as stated in parshas korach with regards to the pans used for the ketores by the 250 heads of the assembly, that were then used as a coting for the mizbeach (alter) as they could only ascend in sanctity. see Menachos 99a.:

ומנא לן דאין מורידין אמר רבי דאמר קרא (שמות מ, יח) ויקם משה את המשכן ויתן את אדניו וישם את קרשיו ויתן את בריחיו ויקם את עמודיו ומנלן דמעלין אמר רבי אחא בר יעקב דאמר קרא (במדבר יז, ג) את מחתות החטאים האלה בנפשותם ועשו אותם רקועי פחים ציפוי למזבח כי הקריבום לפני ה' ויקדשו ויהיו לאות לבני ישראל בתחילה תשמישי מזבח ועכשיו גופו של מזבח

Therefore clothes used for Shabbat clothing are not Kodshim which is osur (prohibited) for benefit rather Chullin which is permitted for benefit. In fact you can even wear your shabbat clothes in the week as it says in Pesachim 112a אמר ר"ע עשה שבתך חול ואל תצטרך לבריות make your Sabbath like a weekday rather than requiring aid from people.