https://dafyomi.co.il/nedarim/tosfos/nd-ts-041.htm
But we have already established that mudar may yes provide an apple to madir, provided that the latter pays for it. So how is this any different?!
madir is not obligated to pay for apple, and may receive it as gift from mudar -- but madir can yes optionally pay mudar for the apple. So question still stand: Why is it forbidden for madir to pay mudar-doctor?
Thank you for your question. I'm not sure your hypothesis that it is permitted to buy from the Madir is correct. As far as I know, the Gemara (end of Perek 3, 31b, recorded as Halachah by the Rambam and Shulchan Aruch) seems to say that in a Zevina Mitza, a sale that both sides are interested in, it is prohibited both ways, regardless of whether the Mudar is the buyer or seller. If the owner of the sick animal would overpay, it would probably be permitted. But if the veterinarian works at regular rates, it is considered as if the animal owner received Hana'ah from the Madir.
Please correct me if I am making a mistake.
Kol Tuv,
Aharon Steiner