Instantaneous approximation for a gauge theory with dressed vertices

We consider a Bethe-Salpeter kernel consisting of one gluon propagating between two dressed quark-gluon vertices. In order to derive the corresponding nonrelativistic potential to $O\left({\left(\frac{v}{c}\right)}^2\right)$ we find that it is necessary to choose a special gauge in which the time-time component of the propagator is instantaneous (to that order). The resulting Fermi-Breit potential including an anomalous magnetic moment is presented and compared with previous results.