DC Circuit Analysis from Sergio Franco Book : Electric Circuit Fundamentals
Notice that to get power equal to \$P = 0W\$ for a current source. We need to have \$P = 2A * v_B = 0W\$
Therefore \$v_B = 0V\$ becouse now \$P = 2A * 0V = 0W\$.
This also implies that \$I_1 = 0A \$ and \$I_2 = 2A\$.
Now it should be easy to see why the answer is \$v_S = 140V\$