Even if PSG and Cavani somehow accept, we wouldn't do it. Financially it wouldn't make sense and after all Pogba's potential sell would be for pure financial purpouses.
Let's say we get Cavani on 4/5 year contract, he would be 31/32 by the time his contract expires and with no re-sell value, or keep him for 2 seasons and sell him and look for his replacament who would probably cost more since no one would offer crazy money for a 29yo Cavani by then.
Or try and keep Pogba for 2 seasons more in which he might offer less than Cavani but still sell him for over 70mln.