He has already agreed to join Barcelona according to Fabrizio Romano

Between today and tomorrow, all the bureaucratic parts will be resolved and the documents will be signed for the transfer of Andreas Christensen.
The player will sign a 5-year contract. In these next days, new contacts will take place between the parties to seal the deal fully. Once the deal is finalized then it will be officially announced.
After closing Christensen’s deal Fc Barcelona has started discussions over a potential proposal for Mazraoui, like his salary and other details – they’re working on it.