  The muffin-tin model of an effective-mass electron interacting with magnetic
ions in semiconductors is extended to incorporate electrostatic potentials that
are present in the case of Mn-based III-V compounds (${Ga}_{1-x} {Mn}_x {N}$,
${Ga}_{1-x} {Mn}_x {As}$). Since the conduction band electron is repelled from
negatively charged magnetic ions and attracted by compensating donors, the
\emph{apparent} value of the s-d exchange coupling $N_0 \alpha$ is reduced. It
is shown that the magnitude of this effect increases when x diminishes. Our
model may explain an unusual behavior of electron spin splitting observed
recently in those two materials in the Mn concentration range x <= 0.2%.
