I think the agent's plan was for him to sit out entirely - no football at all at any team for a year - so he can go to Leipzig next season on a free.I see that there is talk that he'll sit out the season in Barcelona B if he doesn't move to Spurs (or anyone else for that matter) this window. I'm not sure he actually can.
"B" teams in Spain are teams in their own right, with their own squads. They're not "the reserves" as the idea is understood in Britain. They take part in the normal league structure, are promoted and relegated like any other club. The only difference is that if, for example, Barça B won promotion from the second tier, they wouldn't go up and their place would go to the next team down the list. Similarly, if an A team gets relegated and the B team is in the division below, then that team also has to drop down a tier. You can't have an A and B team in the same division in other words. B teams don't play in the Cup either.
Unlike elsewhere, A team players returning from injury or who aren't being picked for the main squad can't drop down and play with the B team to recover or retain fitness, as they can in Britain. The A team can pick players from the B team, but after a certain number of games with the main team, they are deemed to be part of that squad and can't return to the Bs. Also, B players over 23 can't be picked for the As.
I'm not sure what the maximum number of games a B player can play in the A team before they can't return, for some reason I have the number seven in mind, but that may be bollocks or the rule might have changed. I'm pretty certain it's less than 14 though. So if I'm right, young Ilaix wouldn't be able to return from whence he came and "play with the reserves". If he wanted to play competitive football, he'd have to go out on loan to run his contract down and get his (dad's) dream move to Leipzig, his boyhood club.
That seems...not ideal for an 18 y.o.'s development.