Donor is an odd word, less commonly used than the other options, but it just means the person giving the power of attorney to someone else:
"The person authorizing the other to act [by means of a power of attorney] is the principal, grantor, or donor (of the power)."
https://en.wikipedia.org/wiki/Power_of_attorneySo PhB's "mandant" is correct. And yes, Asker, you would use "mandant" in both sentences, but in the second, remember to be precise with your translation of "for." It can be translated as "pour" but in this case it would be much clearer to say "pour le compte de."