We are up to Mishnah 11 in chapter 4 (in some versions of the Mishnah it is numbered 4:9), which records the words of Rabbi Yonasan. Rabbi Yonasan is a Tanna about whom not much is known, neither about his personal life nor about the events of his times. He lived in the second century CE, during the period of Hadrian's decrees, and because of those decrees he left Eretz Yisrael, though in the end he returned to it out of his love for the Land.
This is what he says: "Kol hamekayem es haTorah me'oni, sofo lekayemah me'osher; vechol hamevatel es haTorah me'osher, sofo levatlah me'oni" - whoever fulfills the Torah out of a state of poverty, or because of a state of poverty, will merit in the end to fulfill it and keep it out of a state of wealth. And conversely, whoever does not fulfill the Torah out of his wealth and because of his wealth will in the end lose that wealth as well and come to poverty.
The Source of the Idea in the Verses of Rebuke:
The focus of the Mishnah is fairly straightforward, and its basis is a verse in the Torah - in Parashas Ki Savo, in the tochachah. This is probably the best known line in that passage: "Because you did not serve Hashem your G-d with joy and goodness of heart when you had an abundance of everything," and therefore "you will serve your enemies" - and the verse goes on to describe that this will be "in hunger and in thirst and in nakedness and in want of everything." Frightening words, and their essence is this: whoever does not serve Hashem while he has the ability to do so, and does not rejoice in that service, will in the end lose that very ability. This is the foundation of our Mishnah.
What Is the Mishnah Discussing - Study, Observance, or Support?
First, it should be noted that this does not necessarily refer specifically to Torah study. Some explain the wording as referring to study, and the Rambam indeed speaks explicitly about talmud Torah - that is, whoever studies Torah out of poverty will in the end be saved from that poverty and will continue his studies not out of poverty. But one can also explain "hamekayem" - as referring to the actual performance of mitzvos, and one can even explain it as referring to supporting Torah, such as maintaining yeshivos. There is, then, room here for a range of interpretations.
A Careful Reading of the Rambam:
The Rambam in Hilchos Talmud Torah (chapter 1, halachah 8) writes that every Jew is obligated in the mitzvah of talmud Torah, "whether poor or wealthy." The Rambam understands that there are two reasons why a person's financial situation might make it difficult for him to learn Torah: the poor man is distracted from his learning by the need to support himself, and the wealthy man is distracted by his many business affairs. And from his wording it appears that the greater challenge, the more difficult situation, is specifically wealth rather than poverty - one whose business dealings are complicated finds that they distract him and eat away at his time.
Although this may sound counterintuitive, it is not merely a nice inference in the Rambam's words but a fact of life. If a simple sociological survey were taken among the people of Monsey or Bnei Brak, comparing those living in poverty with the extremely wealthy, one can assume that it is precisely those living in poverty who invest more hours in talmud Torah than the extremely wealthy do. This is an important point that the Rambam raises there in Hilchos Talmud Torah.
If Wealth Is an Obstacle, Why Is the Poor Man Rewarded With It?
Seemingly one should ask: if wealth is such a great obstacle, why should the poor man be granted wealth as a reward for fulfilling the Torah? The answer is that when the Mishnah says that one who fulfills the Torah out of poverty will in the end fulfill it out of wealth, it does not mean that he will become a millionaire. It means that he will be freed from those difficulties that stand in the way of the Torah study of someone who is destitute. He will not be enormously wealthy, but he will not be hungry and he will not be gripped by the pressure and despair of making a living. And the same is true in the opposite direction, regarding the wealthy man who is distracted from his learning.
A Careful Reading of the Mishnah's Wording - the Letter Mem:
Anyone who studies the Mishnah carefully will notice a linguistic difficulty here: the Mishnah uses the prefix mem - "me'oni", "me'osher" - rather than the prefix beis, which would fit better as a description of a state (in poverty, in wealth). Two approaches are offered in explanation:
According to the Tosafos Yom Tov: the mem means "because of," that is, it identifies the cause. In other words, the person is wealthy, and his wealth has left him with complicated business affairs - he has to deal with his lawyers, his accountants and his real estate agents, and as a result his attention is drawn away from Torah study.
According to Rabbeinu Yonah (and this also appears to be the view of the Rema and of the Commentary on the Mishnah): the mem means "out of," out of the mitzvos themselves. In other words, this is not an indirect cause but a direct one: because the person is wealthy, he spends more time on his ski vacations, on his three course meals and on the desserts that are never missing from his table. The life of luxury and the lavish lifestyle themselves cause him to invest less time in keeping the Torah. And therefore the consequence is that the Holy One, Blessed is He, will remove his wealth from him, rachmana litzlan.
A difficulty from real life:
A basic and obvious question arises here: in the real world we all see poor people who study Torah and keep it all the days of their lives and remain poor until their very last day. And by the same token we see wealthy Jews, extremely wealthy ones, who neither keep the Torah nor study it, and who leave this world even wealthier than they were. If so, what is the plain meaning of the Mishnah?
One answer is that the Mishnah always speaks in general rules, while in any particular case there may be additional factors at work:
A person may have withstood the test and not allowed his poverty to interfere with his Torah study, but the Holy One, Blessed is He, sees that if he were to become wealthy he would lose that level - and so He protects him and leaves him in his present situation.
It is possible that he has other sins on his account, which prevent him from meriting both worlds at once.
And by the same token, the wealthy man who was lax in keeping the Torah - perhaps he has no share in the World to Come, and every good deed must receive its reward; and if he does not receive his reward in the World to Come, he receives it in this world.
There are, then, many reasons why this may happen, and therefore, although the rule is true in general, there can be many exceptions to it.
A completely different approach:
Some explain that the wealth and poverty spoken of in the Mishnah are not financial at all, but rather the impoverishment and enrichment of the mind in terms of wisdom. According to this, the Mishnah is speaking of a person who begins his path in struggle and deprivation, and ends up rich in wisdom; and conversely, a person blessed with abundant intellectual resources who does not use them properly will ultimately lose his intellectual abilities.
In summary: although this is a different approach to understanding the Mishnah, the general idea is one and the same: both wealth and poverty present tests, tests that differ from one another, and a person must do his very best to overcome them - whoever he may be - and to study, support and uphold the Torah. If he does not do so, his situation may change for the worse; and if he does do so, with God's help his situation will change for the better.