It has looked like a done deal for months. This just further confirms it. Only more absurd demands from Rabiot's mother could mess this deal up as I see it.
I would actually not be against buying him for a small fee (there has been talk of 5 million euros) next month. His early arrival might be one of the very few excuses available for Valverde to actually rest Rakitic and Busquets more. That alone would make the transfer worth it. Likewise the sign-on-bonus would be smaller.
It probably depends on whether the club can sell Denis Suárez next month. Rafinha is out for the reminder of the season. Samper's contract will expire in July and Valverde does not count on him at all. All 3 could relatively easily find themselves without an actual first team squad number for the reminder of the season if the board wants that to occur.
Rabiot arriving next month would give him additional time to acclimatize and make himself familiar with the club, teammates, playing style, coaching staff, the city etc. However it might just be 1 midfielder too many so unlikely to occur but the idea is not a bad one in practice if he is already nailed on arriving in the first place.
A smart and visionary board (which we have not had since 2010) would have sold Rakitic to QSG for 80-90 million euros (or more or a little less) last summer and bought Rabiot or someone else as his replacement last summer or this January. Or De Jong last summer and Rabiot this January. In such I case there would be space for both in the squad and we would have earned a big sum of money for a 30 year old and declining Rakitic. Alas. If pigs could fly and the rest.