I have commented on Mina earlier in this thread when he was first linked to Everton. I was not a huge proponent of his sale but understood very logically that it was a sound business decision to sell him at the time due to his good WC performances. He was at Barcelona for less than 6 months, a mixed bag in terms of success/fit but I liked his personality, enthusiasm and intangibles. CB is a difficult position to come and establish at Barcelona in under 6 months, especially midway through the season. Clearly Valverde lacked trust in the player and it has to be viewed as a great bit of transfer business for Barcelona (Bought for 11.8 M, sold for 30.25M + 1.5M variable).
Plus, apparently there is a buyback clause of some kind in the contract