Indicate the currents in all the branches using charge conservation as shown in the figure. Applying loop rule, $-\Delta \varphi=0$ in the loops $1 \mathrm{~A} 781,1 \mathrm{~B} 681$ and $\mathrm{B} 456 \mathrm{~B}$, respectively, we get
$\xi_{0}=\left(i_{0}-i_{1}\right) R_{1}$
$i_{3} R_{3}+i_{1} R_{2}-\xi_{0}=0 \quad$ (2) and
$\left(i_{1}-i_{3}\right) R-\xi-i_{3} R_{3}=0$
Solving Eqs. $(1),(2)$ and $(3)$, we get the sought current $\left(i_{1}-i_{3}\right)=\frac{\xi\left(R_{2}+R_{3}\right)+\xi_{0} R_{3}}{R\left(R_{2}+R_{3}\right)+R_{2} R_{3}}$