Is it permissible for one upon whom zakat is due on his sheep to give a ewe on its behalf, then sell this ewe before the poor take possession of it, after they have authorized him to sell it and pay them its price?

A poor person does not own Zakat except by taking possession of it. It is not permissible before possession to authorize someone else to dispose of it, because he does not yet own it. One of the conditions for the validity of an authorization is that the authorizing party must own what he is authorizing concerning.