Jacob’s Pledge
Jacob was fleeing from the wrath of his brother Esau and on his way to Charan he falls asleep. He has an incredible dream: a ladder reaching up from the earth to the heavens, with angels going up and down. When he awakens, he realizes he had slept in a holy place.  It was the site of the future Home of G‑d, the Bais Hamikdash, the gate of heaven. He dedicates a monument to G‑d in that very place and makes a vow:
If G‑d will be with me, and safeguard me on this road that I am traveling, and He will provide me with bread to eat and clothes to wear—
And I will return in peace to my father’s house; and G‑d will be my
G‑d --
And this stone, which I have erected as a monument, shall be the house of G‑d; and all that You give to me, I shall tithe to You.
A literal translation of Jacob’s pledge to tithe to G‑d yields the following translation: “and all that You give me, tithe, I shall tithe to You.”
Double Expression: “I’m Serious” 
Why did Jacob use a double expression, repeating the word tithe?
We can derive a simple answer, based on the commentary of the Radak.  In Biblical Hebrew, known as Lashon Hakodesh [the Holy Tongue], a repetitive expression is used to reinforce the base statement.  When we want to signal that we are serious about a pledge we repeat the word that conveys our commitment.
The need to emphasize our commitment is particularly apt when we are in a vulnerable state. Jacob was destitute when he made his tithe pledge. He had just fled from his brother’s wrath, taking only a walking stick. It is understandable that he would seek Divine protection and support at such time. A dispassionate observer could have been excused for thinking that Jacob’s enthusiasm about tithing to G‑d, expressed at a time when he had virtually nothing, would cool off after he found himself settled and wealthy.
To allay any such concern, Jacob repeated the words “tithe, I shall tithe” to underscore that he was completely serious about his pledge.
Double Expression: A Sign of Affection
Another approach, based on Rashi’s earlier comment, is that the Torah uses a repetitive expression as a sign of affection. Earlier in the Torah, the angel of G‑d called out to Abraham as he was about to sacrifice Isaac: “Abraham, Abraham.” Rashi explains that repeating Abraham’s name was an expression of affection.
Here too, it may be suggested that Jacob wanted to emphasize that he was not pledging a tenth of his fortune to G‑d simply as a way of asking G‑d to get him out of a terrible jam.  Instead, he meant to signify that he was enthusiastic about this donation.
Double Expression: Two Tenths
The Talmud provides a different explanation for the repetitive expression based on Jewish law. The Talmud (Kesubos 50a) states that the obligation is to give no less than ten percent of our income (after taxes) for Tzedakah. Preferably, however, we should try to give a fifth of our income for Tzedakah. This, the Talmud states, is based on the verse in our parsha where Jacob pledged a tithe twice, the basic meaning of which was to give a tenth of his earnings to charity.  With his double statement Jacob paved the way for the idea that those who are more magnanimous should give 20 percent. 
If the minimum is ten percent, and the preferred amount is 20 percent, what about someone who wants to give more than 20 percent? Is there a maximum amount one is permitted or encouraged to give?
The Talmud addresses this directly and states, “One should not extravagantly distribute more than one fifth [of one’s property to charity].”
According to some sources, the rationale for this cap on charity is the fear that  some people will give away too much of their income and impoverish themselves.  That extravagance would leave them dependent on charity, leading to a ludicrous cycle of giving and then taking.
Maximum Qualified
But even this maximum rule has been qualified.
Maimonides, in his commentary to the Mishnah (Peah 1:1), states that one who wishes to conduct himself in the manner of a chosid, i.e., one who goes beyond the basic requirements of Jewish law, may give more than a fifth to tzedakah.
A second qualification is provided by the 18th century sage Rabbi Yaakov Emden (known as the Ya’avatz), who writes in his notes on the Talmud that the restriction on giving more than a fifth share to Tzedakah does not apply to the wealthy. Ya’avatz garners support for this conclusion from a story in the Talmud (Kesubos 63a) about Kalba Savua, the wealthy father-in-law of Rabbi Akiva, who gave Rabbi Akiva half of his wealth.
This, a contemporary author writes, is actually hinted in the verse where Jacob pledges to give a fifth of his wealth to G‑d. We should note that his pledge is preceded by asking for his basic needs: “Bread to eat and clothes to wear.” From this we see that Jacob’s self-imposed limit to giving a fifth was predicated on G‑d granting his request for his basic needs. From this we can infer that Jacob was intimating that if G‑d would give him wealth he would not limit himself to one fifth.
Giving with Joy
There is third exception to the “no more than one-fifth” rule. The Ba’al Shem Tov stated that it only pertains to someone who is by nature a miser and who will feel that he is robbing himself for the sake of the Mitzvah. A person who is generous by nature, and enjoys giving, may give more than a fifth because he or she is inclined to do so out of genuine generosity, with joy.
Tzedakah for Teshuvah
There is a fourth recognized exception to the limit of one fifth for Tzedakah. The Alter Rebbe (Rabbi Schneur Zalman of Liadi, founder of the Chabad movement, the anniversary of whose liberation from Czarist imprisonment we will soon celebrate on the 19th of Kislev) in his classic work, the Tanya, discusses the various approaches to Teshuvah (repentance, return). One of the means to cleanse oneself from sin and self-refinement is through Tzedakah. The Alter Rebbe states that one need not heed the admonition to limit himself to one-fifth. Just as we do not restrict the amount of money we may spend for our health, there is similarly no restriction on the amount one may give to Tzedakah for our spiritual health.
The Limits of Tzedakah Today
While these qualifications were always valid, there is an even a greater need for taking “advantage” of these exceptions to the one-fifth rule in the present day and age.
To understand the reason for the emphasis on giving beyond the traditional limits in today’s day and age, as we stand on the threshold of the Final Redemption, we must cite the Talmudic statement that Tzedakah is the Mitzvah that most hastens the Redemption.
One way of understanding the connection between Tzedakah and Redemption is that G‑d rewards us measure-for-measure. If we perform the mitzvah of Tzedakah, G‑d reciprocates and performs the Mitzvah of Tzedakah by redeeming us from exile.
When we discuss Redemption we must recall how the Alter Rebbe distinguished between giving for the sake of the Mitzvah of giving Tzedakah and for our own personal redemption from sin. Personal Redemption is best accomplished by exceeding both tithing limits of a tenth or a fifth. 
If one’s personal redemption is facilitated by giving more than a fifth, how much more so can it be said that the Final Redemption of the entire world necessitates going beyond the limits that Jewish law imposes?
Two Scenarios for Moshiach: Two Levels of Tzedakah
Moreover, the Talmud describes two scenarios for the coming of Moshiach: If we are found lacking it will happen in a totally natural way; if we are meritorious it will happen in a supernatural way. Moreover, if it happens in the natural manner, Redemption may take place with much travail and hardship. If it happens because we are meritorious Redemption will occur in a most peaceful and positive fashion.
These different scenarios depend on the way we act, and more specifically, the way we perform the Mitzvah of Tzedakah.
To be sure, when we give Tzedakah in its most “natural,” prescribed manner, by not exceeding a tenth or a fifth of our income, we will certainly deserve, bring and hasten the Redemption. However, if we exceed those limits, especially when we do it with great joy and enthusiasm as the Ba’al Shem Tov advocated, G‑d will certainly reciprocate and perform His Tzedakah by redeeming us in a way that transcends all bounds; free of all travail, constraints and obstacles.