I have scalar field mu and vec
 

I have scalar field mu and vector field A (solving 3D magnetic field).

I want to set boundary condition on patches inside solved domain where mu is changing from one value to another value in step.

Vector field A should fullfill condition
dA/dn|l - dA/dn|r = mu_l/mu_r (derivative in direction of face normal from left and right side)

I have following questions:
1. Is it possible to set it as a boundary condition (condition is inside solved domain, not on boundary)?
2. Which boundary condition should I modify to do it.

Thank for any advice

Jakub Zlamal