At 20m it seems too good to pass up.
I actually like this deal more and more because I've heard Gladbach fans say he can play defensive midfield like Rodri, but has also played well as a center back, and you know how Pep likes players who can play in multiple positions.
Considering we often shift to a back 3 also, I can in a weird way see this as a long term Walker replacement since he plays on the right side of our back 3 (as well as being Rodri's backup).
Less players who play more positions means more minutes for all.