R. Yitzchak Sheilat, Iggeros Ha-Rambam, vol. 1 p. 229n:
It seems from here that the Rambam was an actual partner in his [brother's] merchandise... And through this [R. David his brother] enabled him to "sit in peace" and study Torah. Perhaps it is specifically in this fashion, and not as a gift, that the Rambam understood the idea of Yissachar-Zevulun and Shimon the brother of Azariah, that are mentioned in the Midrashim of the Sages (see Bereishis Rabbah 72:5, 99:9; Vayikra Rabbah 25:2)... However, from the base law it is permissible [to support someone studying] even entirely for free [without a partnership], since the initiative to this arrangement comes from the brother who supports. However, as a pious practice the Rambam was a partner in his [brother's] property.
Ibid., p. 193n:
In the Commentary to Avos 4:7 it is explained that one should differentiate between someone learning Torah accepting support from others, which is prohibited, and assistance to Torah scholars in supporting themselves in a manner in which they will not need to spend much of their time, such as helping them invest their money or sell their wares, which is a desirable matter... The permission is that "these actions are done by some sellers to others out of respect even if there is no wisdom, and a Torah scholar can at least be treated like a respected ignoramus."
Ibid., p. 311n:
The Rambam agreed that "pay for not doing something else" (sekhar batalah) is permissible, as explained in Kesuvos 105a and as he ruled in Hilkhos Sanhedrin 23:5. Included in "sekhar batalah" is when he has an occupation through which he supports himself and others come to him and ask: "Teach us Torah or rule for us full-time and we will pay you a defined salary for your absence from your work." And so he ruled in Hilkhos Shekalim 4:7, that the judges of thefts in Jerusalem would receive a defined salary from communal funds. What the Rambam considered to be forbidden and a desecration of God's name is when the initiative to receive a salary or stipend from teaching Torah comes from the Torah scholar himself, which is the making of Torah into "an ax with which to dig" and a descent of it to one of many occupations, or even lower. However, it is preferable to refuse entirely a salary for [teaching] Torah, even in a manner in which it is permissible and even on the initiative of others...