Question here is if it's really worth to pay 20m for a player whose contract expires in 12 months.
If we want him, yes. Someone will pay it, and unless Alvarez badly wants to play for us there's little room for negotiation. At some point River Plate would just keep him for another year.
If we (or no one else) will manage to lock him up soon or simply pay his release clause before January, River not only won't mind to keep him, they're actually actively trying to extend his contract and update his release clause from 25 to 40M. So yes, if we want to get him for 25M or less, we need to act now.