The problem of anisotropic radiation transfer in a finite slab with reflecting boundary conditions and an internal source (problem 1) is solved in terms of the corresponding problem with isotropic boundary conditions and a free source (problem 2). Two exact relations for the partial radiation heat flux at the boundaries of problem 1 are obtained in terms of the reflection and transmission functions of the isotropic boundary problem (problem 2). Therefore, if exact values of the albedos are known, then the partial radiation heat flux is calculated exactly. The advantages of our method are the following: (i) the relations are algebraic in the albedos and (ii) the internal source term appears explicitly in these relations and there is no need to look for particular solutions for problem 1.