וְכל אֲשֶׁר תִּתֶּן-לִי, עַשֵּׂר אֲעַשְּׂרֶנּוּ לָך
In this Posuk (28:22) Yakov exclaims his intent to tithe all that he receives from H’. The double-use of the word tithe is detected by the Gemora in Kesuvos 50a, which learns from here that when giving charity, we are only required to give away up to a fifth of our yearly earnings, המבזבז, אל יבזבז יותר מחומש, (two-tenths = a fifth!).
The Ra’avad explains that the reason for this is because if a person were to give more than this, he may eventually find himself unable to support his own financial requirements. (If, however, someone is in a particularly healthy financial situation, there is no restriction on how much he may give away to charity.)
The Gemora in Bava Kama 9a appliesthe principle learned here to all Mitzvos in general. There is a well-known concept that beyond the cost of doing a regular Mitzvah, one is required to spend additional money for הידור מצוה , beautifying the Mitzvah. The Gemora in Bava Kama, based on the principle learned from our Posuk, restricts the amount one should spend on הידור מצוה to an extra third on top of the original price for the regular Mitzvah.
Rashi explains as follows. Say a person had the option of spending £100 on a nice Esrog. He then saw an even nicer Esrog. Provided that the second Esrog cost no more than £133, he is required to purchase the second Esrog.
Tosafos explains the Gemora a little differently. The minimum size for a kosher Esrog is the size of an egg. If one found an Esrog exactly the size of an egg and then came across another Esrog that was bigger than this minimum sized one, he is required to spend up to a third more in order to buy the larger Esrog. The rationale for this is that we are worried that the small Esrog may shrink over Sukos and would subsequently be unfit for the Mitzvah.
(The Pnei Yehoshua clarifies the point of contention between Rashi and Tosafos. Tosafos was unwilling to explain like Rashi because according to Rashi, the Gemora would seem to require us to spend an extra third of the Mitzvah every time we find a nicer Esrog. The first day he may find an ordinary one for £100 and on the second day he may find a nicer one which he would have to buy for £133. If on the third day he subsequently found a third Esrog nicer even than the second, he would have to pay up to £177. If on the fourth day he found a nicer one still, he would have to pay £236 etc. etc. This would seem to be illogical. Therefore, Tosafos explained the Gemora in a way that could be understood as providing a one-time obligation to upgrade; only where the original Esrog was small, and the second was large.)
The Shulchan Aruch §656:1 mentions the opinion of Tosafos and then brings an anonymous opinion that seems to be like Rashi. The rule is when learning Shulchan Aruch that whenever a second opinion is brought anonymously, we do not rule in accordance with that view. The Rem”a adds that as opposed to positive Mitzvos מצוות עשה, where the principle applies, with negative Mitzvos לא תעשה, we are required to spend as much as possible in order to avoid transgressing a Mitzvah. The Mishna Brura §9 explains; since transgressing a negative Mitzvah involves an actual expression of going against the word of H’, we need to take all measures to avoid doing so.