The agreement stipulated that in the event an offer was accepted for any of those players, Barca would be given a certain amount of time to match the fee Santos had agreed with any other club.
However, Santos failed to properly notify Barca when they accepted a bid worth around €25m from Inter Milan for Gabigol in 2016.
Sources told ESPN at the time the Catalan club were unhappy they were only informed of a potential move to Inter with just 48 hours remaining in the transfer window.
Barca's initial complaint was dismissed by FIFA but on Thursday, over four years after Gabigol's move to Italy, the Court of Arbitration for Sport (CAS) ordered Santos to pay €2.9m for breaching the agreement between the two clubs.
It is the second time the CAS has sided with Barca following a string of disputes relating to Neymar's 2013 move to Camp Nou.
Gabigol, now 24, scored just once in 10 appearances during his only season with Inter. Loan spells followed with Benfica, Santos and Flamengo, where he improved dramatically and scored two goals to help the club win the Copa Libertadores in 2019 before making the move permanent.