References for not receiving alms in exchange for Dhamma teaching 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".
An extended answer would probably result in getting aware that there are very, very less who do not maintain just a livelyhood in using the tripple Gems. Less who are actually not in the whole of a serious transgression, with less hope to find a way to get reconciled. While such would formally just touch those "bond" to Vinaya, from a kammic view it touches all of the many, regardless of their clothes.

An that is why even laypeople have been taught the matter, only traceable for those looking sometimes inwardly.


  One  should not make an effort everywhere,
  
  should not be another's hireling,
  
  should not live dependent on another,
  
  should not go about
  
  as a trader in the Dhamma.
  
  ud 6.2


One would even find the message in the Jatakas, that not even Gods and Devas practice generosity in exchange.

One should also not underestimate that there are four kinds of nutritions in this regard.

Turn on satipaṭṭhānā and you may face corruption and how it arises in one self, and then is no more need to seek Dhamma-Vinaya outside. Till there one is wise to listen and follow the Elders and those ahead in regard of letting go.

(furthermore: Read Iti 107 as what it actually says, without assuming anything in addition)

[Note: this is a gift of Dhamma, not meant for any worldly trade and exchange]