The Serie A side are now considering their legal options after feeling as though they were forced into an "auction" despite believing negotiations were complete.
Malcom, a Brazil youth international, described his sudden switch to Barca as the fulfilment of a boyhood ambition.
"This is a unique experience. I feel really happy. It's a dream come true to come to one of the biggest clubs in the world," he told Barca TV.
"I hope to make the fans happy. I'm really looking forward to meeting my team-mates, to talk to them and find out more about the way we play.
"I'm really excited and in a hurry to get started. It's difficult to explain the feeling. It's a dream come true since I was young."