Do either Mi Shepara or Mechusar Amanah apply to backing out of a purchase with a Goy?
Answer:
The Tzelach (Pesachim 31) writes that the principle of Mi Shepara applies even to a non-Jew. There is room to debate this idea based on the question of whether money is a Torah acquisition concerning a non-Jew or not.
Concerning Mechusar Amanah, the Gemara (Bechoros 13b) writes that the principle does not apply.
Best wishes.