Mina joined the Toffees for an initial £27m with a further £2m in add ons, but if they choose Barca will be able to re-sign the defender for £53.9m.
This is not an unusual clause, particularly involving clubs from La Liga, but does mean that the Premier League side face the prospect of losing the 23-year-old should the Catalans believe he has considerably improved.
While Mina had no future in the first team at the Camp Nou in the short term, there is a feeling there that the Colombia international could greatly improve in the future.
The Merseysiders will double their money in this scenario of course, yet will be acutely aware that if the centre-back fulfills his great potential then they would be powerless to keep him at Goodison Park should his former employers make a move.
Mina only joined Barca in January from Brazilian side Palmeiras, but failed to truly impress before his stock greatly rose over the summer after impressing for Colombia at the 2018 World Cup.