In the reaction ${\mathrm{HAsO}}_2+{\mathrm{Sn}}^{2+}\to \mathrm{As}+{\mathrm{Sn}}^{4+}+{\mathrm{H}}_2\mathrm{O}$ ;    oxidising agent  in the reaction is

Oxidizing agent itself, undergoes reduction during a redox reaction

${\mathrm{H}\stackrel{+3}{\mathrm{As}}\mathrm{O}}_2+\stackrel{+2}{\mathrm{Sn}}⟶\stackrel{0}{A}s+\stackrel{+4}{\mathrm{Sn}}+{\mathrm{H}}_2\mathrm{O}$

Hence, here HAsO₂ is acting as oxidizing agent.