Talk is that with Barcelona stating they want all the money up front, it will cause PSG to breach Financial Fair Play. So, it might not be a done deal yet. If it is Barcelona playing games as I have heard suggested, I don't understand why. If Neymar is going to leave, get rid of him ASAP so you have the cash to go out and find a replacement or two. Why drag it out?
As for a replacement, why not Mbappe? Real Madrid apparently have interest, but Barcelona would now have the money to go get him immediately. Heck, if I am Arsene Wenger, I am calling Barcelona telling them they can have Sanchez (who will most likely not sign a new contract at Arsenal no matter what). The only catch I would have is once sold, Barcelona could not sell back to any Premier League club.