Clubs are blocked from bringing in new players unless there are very specific circumstances or the player in question is a free agent.
The latter could be the case for Hansi Flick, amid rumours he is ready to challenge Inter Milan and Juventus for Brazilian defender Dodo, after he rejected a contract extension at Fiorentina this summer.
UEFA rules allow clubs to start free transfer talks with players from other leagues to the one they are registered in at the start of their final six months under agreement.
Barca could kickstart informal talks in the coming weeks and agree a formal deal in January.
It would then be a decision between the two clubs to let him move on early and Barca may have to pay a settlement fee to tie up the end of his contract.