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With strong spin orbit coupling, 5d transition metal ions in a d⁴ (t_2g⁴e_g⁰) configuration are expected to be non-magnetic with all 4 electrons in J=3/2 shell leaving j=1/2 empty. This unique feature of 5d⁴ ions can be employed to dilute the magnetic sublattice or even switch on/off the magnetism. Here we report the temperature induced electron transfer from Tb to Ir ions in a double perovskite Ba_2-xSr_xTbIrO₆. This thermally driven valence state transition can be employed to tune the electronic configuration of Ir and thus the magnetic order of the double perovskite. The nonmagnetic Ir⁵⁺ (d⁴) ions dilute the Tb³⁺ sublattice, which impedes any long range magnetic order. The electron transfer from Tb³⁺ to Ir⁵⁺ turns the nonmagnetic Ir⁵⁺ (J=0) to magnetic Ir⁴⁺(J=1), which enhances the Tb-O-Ir-O-Tb exchange interactions and favors long range magnetic order at low temperatures.