It's not clear how the agreement can be activated, but reports in Italy state the Serie A giants insisted upon the clause when they agreed to part with the France midfielder for a then world record £89m fee in August 2016.
Juventus wanted to be at the front of the queue if things didn't somehow work out for Pogba back at United, having quit Old Trafford on a free transfer for the Old Lady back in 2012.
Real Madrid are said to be closely watching the developments, even though the club's president Florentino Perez was unimpressed with Pogba for previously choosing United over the 12-time European Cup winners.
It could even see Juventus try to buy Pogba back at the discounted rate, before then selling him on to the Bernabeu giants for an instant and substantial profit.
That would certainly seem a worst-case outcome for United execs, who are increasingly concerned at a perceived breakdown in the relationship between the player and manager Jose Mourinho.
Pogba has met twice in the last seven days with the Portuguese tactician to discuss his misfiring role in the team, with the 24-year-old's camp feeling he is being made a scapegoat for some of the Red Devils' poorer showings this term.