Barcelona have been fined 602 euros (£503) following a complaint by Atletico Madrid over contact the club made with striker Julian Alvarez.
Atletico accused La Liga rivals Barcelona of approaching Argentina international Alvarez without first notifying them in writing.
The 26-year-old remained at Atletico and has a contract which runs until June 2030.
The fine was issued by the Spanish football association. Atletico also complained to Fifa, but football's governing body declared it had no jurisdiction over the case.
Alvarez has frequently been booed by his own supporters this season but received a standing ovation from some after he scored the winning goal against Deportivo La Coruna on Saturday.
Following that victory, which took Diego Simeone's side within two points of leaders Barcelona, Alvarez took the opportunity to apologise to Atletico's fans.
Referring to an interview he gave during the summer in which he said he wanted to "fulfil his dream" of playing for Barcelona, Alvarez said: "It was a mistake. I understand why the fans are angry.
"If I were a fan, I'd be angry too. Especially given how they've supported me since I arrived."
Alvarez joined Atletico Madrid from Manchester City on a six-year deal in a move worth up to £81.5m in August 2024.
After scoring 36 goals in 103 appearances for City, he has found the net 51 times in 115 games for Atletico.