According to this comment:
I think that monks and laypeople are meant to depend on each other (e.g. as described in Iti 107) however monks must not sell their dhamma talks for food. So there are monastic rules about food and dhamma talks: they're separate transactions ... a quid pro quo is forbidden -- having given a dhamma talk (i.e. after his speaking) the Buddha was no longer willing to receive food, because that would appear as if he were receiving food in exchange for or as payment for talking.
Please provide references from the Vinaya and/or Patimokkha for "there are monastic rules about food and dhamma talks: they're separate transactions ... a quid pro quo is forbidden".