  Experimentally, the intermetallic compound Mn$_{4}$FeGe$_{3}$ has been
recently shown to exhibit enhanced magnetic properties and spin polarization
compared to the Mn$_{5}$Ge$_{3}$ parent compound. The present {\em ab-initio}
study focusses on the effect of Fe substitution on the electronic and magnetic
properties of the compound. Our calculations reveal that the changes on the
Fermi surface of the doped compound are remarkable and provide explanations for
the enhanced spin-polarization observed. Finally, we show that it is indeed
possible to tune the degree of spin-polarization upon Fe doping, thus making
the Mn$_{1-x}$Fe$_{x}$Ge$_{3}$ intermetallic alloy very promising for future
spintronic applications.
