The arrest took place on Monday as the Catalan regional police searched the offices at the Camp Nou Stadium as part of an investigation into an affair known as 'Barcagate'.
The Indian skipper is also the fourth most followed sportsperson on Instagram.
Barca are third in the standings on 47 points, trailing leaders Atletico by eight points and second-placed Real Madrid by five points.
The deal over four years is the biggest signed by any athlete in sports history, says a report in Spanish newspaper El Mundo.
Messi was sent off for the first time in his Barcelona career in the closing stages of extra time as they lost 3-2 to Athletic Bilbao.
The win took Barca above Sevilla and into fifth place in the standings on 28 points, 10 behind leaders Atletico Madrid who beat Alaves 2-1 earlier on and who have a game in hand on the Catalans.
Martin Braithwaite missed a penalty for the Catalans.
From January, Messi will be free to negotiate his next move as he will enter the final six months of his contract with Barcelona.
The Barcelona talisman scored his 644th goal to surpass Pele as the all-time record scorer for a single club.
Messi equalled Pele's record of 643 goals for a single club on Saturday but was unable to help Barcelona beat Valencia in La Liga as they had to settle for a 2-2 home draw.