A representation formula for the distributional normal derivative

We prove an integral representation formula for the distributional normal derivative of solutions of $$\left\{ \begin{aligned} - \Delta u + V u &= \mu && \text{in $\Omega$,}\\ u &= 0 && \text{on $\partial\Omega$,} \end{aligned} \right.$$ where $V \in L_{\mathrm{loc}}^1(\Omega)$ is a nonnegative function and $\mu$ is a finite Borel measure on $\Omega$. As an application, we show that the Hopf lemma holds almost everywhere on $\partial\Omega$ when $V$ is a nonnegative Hopf potential.