The gemara says that one who has enough for today and says "what will i eat tomorrow" is considered on of little faith. This was the aveira that the gemara mentions on 46b when Elisha improved the water and ruined the parnassah of the "ketanei amana" who were worried they would not have what to eat. However, when Rashi explains "anshei amana" - people who have faith, he writes "they spend money on hiddur mitzvah, tzedaka, and shabbos v'yom tov". By each one of these there is a specific guarantee that it will be on Hashems cheshbon. Hiddur mitzvah - baba kama 9b, tzedaka - ta'anis 9a, shabbos and yom tov - beitzah 16a. It is understandable that since Hashem gave a special promise that one will be reimbursed, one should not hesitate to spend money. But, if one has what to eat for today and not for tomorrow why is he counted as "ketanei amana" for being nervous; he wasn't given a divine guarantee? The gemara implies that just as Hashem guarantees on the items listed in rashi, he also gives a havtacha of "umasbiah l'chol chai ratzon" and therefore one is considered ketanei amana if he has for today and is nervous about tomorrow - a pretty high level of faith!