Garcia will return to his boyhood club on a deal until the end of the 2025-26 campaign after three years at City and follows striker Sergio Aguero in making the switch from Manchester to the Camp Nou.
"It's always a great time to join Barca. The president has set us on an upward path, full of youth and experience.
Messi's current contract expires at the end of June and he is free to discuss a move to another club, but media reports on Tuesday suggested an agreement was imminent.
"Talks are ongoing with Messi's people and, as I said, they're going well. We hope to keep making progress. It's not done but we know that Leo wants to stay.
The withdrawal deals a fresh blow to the GSMA industry association that hosts the event, which was cancelled last year at the last minute while this year's edition was pushed back to the end of June to maximise its chances of happening.
"At the present time, we still do not believe it is justifiable to take this risk for a large number of our employees," the Bonn-based group said in a statement issued to Reuters.
UEFA opened disciplinary proceedings against Barca, Real and Juve on Tuesday, prompting the three clubs to issue joint statements saying they would not bow down to pressure from the governing body.
"I've told (UEFA president Aleksander) Ceferin that we aren't going to say sorry. And we don't plan on paying any sanction," Laporta told a news conference on Friday.