Olmo's contract includes a clause that allows him to leave for free if he can't be registered by December 31st, because he wouldn't be able to play for the rest of the season. He would, of course, have to exercise that clause. So Barca can go at this in a couple of very Barca-esque ways. The first is through the pulling of some highly shady, and speculative levers. A tried and tested trick, but one it seems that La Liga is finally closing the door on; and the other is by restructuring his contract. If Barca extend his contract before December 31st, reducing his salary for the year to the amount he has already been paid, he would essentially play the rest of the season for free and be able to be registered. In turn they would pay him the deferred sum plus a bonus next season, or spread over the remaining years of his contract. This is essentially what they did with De Jong. It true kicking the can down the road mode, but we know they don't care about that. And players seems to bend over to take it up the rear from the club.
De Jong is now one of the highest paid players in La Liga, but sits on the bench most of the time, because of how much deferred wages he accumulated. And what did he get in return for deferring the vast chunk of his wages? They desperately tried to kick him out the door before they came due, and in doping so refused to make him whole. He, of course, famously dug his heels in and stayed, but weirdly still preaches untold love for the club that treated him with so much disregard. In a normal world, I would say Olmo would look at that and say, pay me what you owe me this season, or I walk on a free in January. But you all know he won't.