Parshat Ki Tavo tells us about bikkurim -- offering God the first fruit of the harvest. [Deuteronomy 26:2-11] The Rambam says that all donations for the sake of God should be of the best goods we possess:
Everything given for the sake of the Almighty should be of the most attractive and highest quality. If one builds a house of prayer, it should be more attractive than his own dwelling. If he feeds a hungry person, he should feed him from the best and most tasty foods of his table. If he clothes one who is naked, he should clothe him with his attractive garments. Whenever one designates something for a holy purpose, he should sanctify the finest of his possessions; as [Leviticus 3:16] states: "All of the choicest quality should be given to God." [Mishneh Torah, Things Forbidden on the Altar 7:11]
The Lubavitcher Rebbe includes in this rule: (1) sacred studies before secular studies in school and (2) if you have two occupations, one to pay the bills and another to benefit the community, give priority to the second.
Both seem to expand "for the sake of God" quite liberally, and don't appear to specify a limit. That bothers me.
My question: Does "giving to God" really apply this far? What if you don't give of the best? It's better than giving nothing. Charity is charity, no? Isn't it natural to save the best for yourself? If I upgrade a possession (a car, an appliance) and give the old one to charity [or to the congregation], is that bad?