You can issue a power of attorney at a foreign diplomatic institution of Ukraine or at a local foreign notary public.
The father may not be present at the certification of the donation contract, but he must write a statement of consent to the notarial act and notarize it.
The sales contract should be notarized. After all, in the event of misunderstandings between the parties and, say, an appeal to the court, it will be the basis for the consideration of the case, and not the deposit.