Question

Can the magnitude of a magnetic field be negative?

If we consider an electron in a mag field,

using r=mv/qB, the negative charge on the electron gives a negative answer. Do I drop the minus sign?

Answer 1

To get this you used the Lorentz force equation, a vector equation, then dropped the vectors and equalled it to a scalar centripetal acceleration as a scalar. Ignore the negative, and make certain you know the direction of curvature from the right hand rules.

Answer 2

If course. B is a vector, it can be + or -

Your equation is a magnitude type equation that does not really do the vector computations correctly to decide what direction r is in.
You have to do that yourself.
You need to go back to the vector equation for force on a charged particle in a B field:
F = q (v X B)
here q = -e
so F = -e (v X B)
figure out the direction of v X B with the right hand rule
Your force on the electron causing it to curve is in the opposite direction.