Rambam on Being Paid to Learn or Teach Torah

This is the first post of a proposed series on the Rambam's position regarding being paid to learn or teach Torah. It is well known that the Rambam was of the view that it is forbidden to receive payment for learning or teaching Torah. However, some ask, did not the Rambam do precisely that? Did he not learn Torah while his brother, David, worked as a merchant and supported him? Only after his brother died tragically did Rambam start working.

The answer seems to be that this is not true.

Joel Kraemer, "Moses Maimonides: An Intellectual Portrait" in Kenneth Seeskin ed., The Cambridge Companion to Maimonides, p. 28:

Biographers have written that as result of David's death Maimonides had to relinquish the life of a scholar and take up medicine as a profession, but there is no evidence for such a transition. Maimonides had studied medicine in North Africa before coming to Egypt and attained prominence as a physician in his early days in Egypt even before David's demise.

On the advice of a scholar, I looked in S.D. Goitein's A Mediterranean Society: The Jewish Communities of the Arab World as Portrayed in the Documents of the Cairo Geniza and found the following about two brothers named Labrat and Judah (vol. 5, pp. 392-393):

The addressee [of a letter], his younger brother Judah, whom Labrat had brought up (probably because of the early death of their father) and for whom he harbored tender feelings (as did Moses Maimonides for his brother David), was later to become a great merchant and renowned philanthropist in Egypt...

It was customary in merchants' families that one member, usually a father or elder brother, stayed put and the others traveled. Labrat and Judah (like Moses and David Maimonides later) had divided the family business between themselves in such a way.

It seems that Maimonides worked together with his brother as well as being a practicing doctor on his own.