I heard Juve is going for De Gea and a couple other United players this January. Could this be an opportunity for Ronaldo (and his agent) to engineer 'a return'? 
Actually, the rumor bounced here in Italy too!
The good part of this rumor is that if he will come here it will be a free player and he will cost 0.
His contract will end next year!
So Juventus can sell Szczesny for 35-40 M https://www.transfermarkt.it/wojciech-szczesny/profil/spieler/44058 and take DeGea for 0.
What's about of salary?
Szczesny as today cost at Juventus around 8m with taxes.
DeGea now gains 11.6 m without taxes from ManU.
If he keeps the same salary for Juventus will be 11.6+25%(11.6)= 14.5 m with taxes.
So basically DeGea will cost 6.5m /y more then Szczesny, but Juventus can take around 40m from the sell.
Now if we do 40/6.5 we will have around 6 years if DeGea comes and signs a contract of max 5 years this operation it will +EV for Juventus.
A very smart calculation from you.
What you say makes sense, As we know that Juventus officials will use every moment to be able to benefit. They have often done things like this and in the end they benefit from every sale of players and even the purchase of players they can also think of profits.