  Magnetic and ferroelectric properties of layered geometrically frustrated
cluster compound FeTe$_2$O$_5$Br were investigated with single-crystal neutron
diffraction and dielectric measurements. Incommensurate amplitude modulated
magnetic order with the wave vector $\bf{q}$=($\half$, 0.463, 0) develops below
$T_N=10.6(2) {\rm K}$. Simultaneously, a ferroelectric order with the
spontaneous polarization perpendicular to ${\bf q}$ and to Fe$^{3+}$ magnetic
moments emerges. The observed ferroelectricity and extraordinary linear scaling
of the ferroelectric and magnetic order parameter are provoked by the striction
of the intercluster Fe-O-Te-O-Fe bridges leading to the shift of Te$^{4+}$ ions
and polarization of their lone-pair electrons.
