  Let $\Gamma$ be a finitely generated torsion-free group. We show that the
statement of $\Gamma$ being virtually abelian is equivalent to the statement
that the $*$-regular closure of the group ring $\mathbb{C}[\Gamma]$ in the
algebra of (unbounded) operators affiliated to the group von Neumann algebra is
a central division algebra. More generally, for any field $k$, it is shown that
$k[\Gamma]$ embeds into a central division algebra in case $\Gamma$ is
virtually abelian. We take advantage of this result in order to develop a
criterion for existence of units in the group ring $k[\Gamma]$. We develop this
criterion in the particular case of $\Gamma$ being the Promislow's group.
