The Pasuk says:
וְשִׁלַּם אֹתוֹ בְּרֹאשׁוֹ וַחֲמִשִׁתָיו יֹסֵף עָלָיו – ….he shall pay its main, adding its fifths to it… (5:24)
Chazal teach us that all expenses are calculated and apportioned from Rosh Hashanah to Rosh Hashanah, with the the exceptions of monies allocated for: – Shabbos, –Yom Tov, –teaching Torah to one’s children, and the Medrash Rabbah adds Rosh Chodesh to the list as well.
The Tzemach Tzvi says that this is alluded to in this Pasuk. וְשִׁלַּם אֹתוֹ- all his money, whatever he pays and whatever he earns, is earmarked בְּרֹאשׁוֹ – on Rosh Hashanah – וַחֲמִשִׁתָיו יֹסֵף עָלָיו – and his fifths will be added to him. What are these חֲמִשִׁתָיו of which which we speak? חוץ מהוצאות שבת, תורה, יו”ט, ור”ח – with the exception of expenses for Shabbos, Torah, Yom Tov and Rosh Chodesh. These will be יֹסֵף עָלָיו – added to him as the year goes on, subject to his actions.
(This is my addition, feel free to disagree)
It is worth pointing out that the וְ of ור”ח is used as part of the initials, rather than the ר of Rosh chodesh, and the end result is that the initials are a perfect reflection of the way (diyuk) the Medrash Rabba explained this concept, that Rosh Chodesh too is part of the list.
(end of my addition)
There is a story told about the Pnei Menachem, who once sent his son to school with an envelope for his Rebbe. The Rebbe refused to accept the envelope, as he did not feel he could take money from the Pnei Menachem. A few nights later, the Pnei Menachem called up his son’s Rebbe, and said that he was obliged to accept it, as זה נהנה וזה לא חסר , (literally, this one gains, and the other does not lose) a mechanism that enables someone to use something that the owner is not using and had no use for, for free. He explained with this Dvar Torah that this money was not normal money and could come into reckoning, as money spent on teaching your son Torah is not included in your “annual salary”. Presumably, the Rebbe accepted the envelope.