Work done by a conservative force is the change in potential energy, therefore, $W=U_{\mathrm{f}}-U_{\mathrm{i}}$

Now, the gravitational potential energy per unit mass or gravitational potential of the particle is, $V=-G\frac{M}{r}$

So, the initial potential energy, $V_i=-\left(6.67\times {10}^{-11}\right)\frac{1000}{1}=6.67\times {10}^{-8} \mathrm{J} {\mathrm{kg}}^{-1}$

The final potential energy to take the particle far away from the sphere is $V_f=-G\frac{M}{\infty }=0$

Work done per unit mass of the particle to take it far away from the spare against the gravitational force is, 

$$\begin{gathered} W=V_f-V_i=0-\left(-6.67\times {10}^{-8}\right) \mathrm{J} {\mathrm{kg}}^{-1} \\ W=6.67\times {10}^{-8} \mathrm{J} {\mathrm{kg}}^{-1} \end{gathered}$$