Identifying Lewis acid
What we have, in the Lewis theory, is an acid-base displacement reaction.  The Lewis acid, $\ce{H^+}$, is initially combined with one Lewis base, $\ce{NO2^-}$.  Then another Lewis base, $\ce{HPO4^{2-}}$, takes the $\ce{H^+}$ away displacing the $\ce{NO2^-}$.

Similarly when $\ce{HCl}$ is dissolved in water and dissociates, the water is displacing the chloride ion.  The Lewis acid involved in this displacement is again $\ce{H^+}$.