The Croatian teenager agreed to a deal to join Gerardo Martino's men from Prva HNL side Dinamo Zagreb in March, prior to the world governing body's decision to ban Barca from participating in the next two transfer windows.
FIFA sanctioned Barcelona due to their apparent illegal transferring of players under 18.
"I cannot foresee for one second that my client, Alen Halilovic, will not be able to play for Barca," lawyer Jean-Louis Dupont told Spanish radio station RAC1 on Thursday.
"This is an operation that has been completed prior to the sanction."
Barcelona president Josep Maria Bartomeu has already flagged an appeal from the La Liga giants.
Belgian Dupont has experience in cases involving high-profile footballers, after helping then RFC de Liege player Jean-Marc Bosman reach French club Dunkerque in 1995 with the player having been restricted due to resistance from both clubs.
Dupont said he felt the sanction handed to Barca was misguided.
"FIFA has done well to identify a problem, by I think they're trying to apply the wrong medicine," Dupont said.
"When this FIFA ban doesn't allow for the future of a child and his family, then the regulation is mistaken.
"I have no doubts, that if Barca were to sign an 11-year-old, they would guarantee proper education and the correct social environment."
