YUSUFALI: O ye who believe! the law of equality is prescribed to you in cases of murder: the free for the free, the slave for the slave, the woman for the woman. But if any remission is made by the brother of the slain, then grant any reasonable demand, and compensate him with handsome gratitude, this is a concession and a Mercy from your Lord. After this whoever exceeds the limits shall be in grave penalty.
Perhaps a line of questioning could go like this:
PG: "So, we see that according to your God, the life of a free man is not equal to the life of a slave. If your God believed the life of a free man was equal to the life of the enslaved man, your God would have simply said, "the law of equality is prescribed to you in the cases of murder: one man for one man". But your God made a distinction between the value of the free man and the value of the enslaved man. Why do you personally believe your God values the life of a slave less than the life of a free man? Is not a slave still a man? Even though he may have been enslaved, he's still a man, and in my eyes, he's every bit as valuable and important as a free man. Yet your God makes a clear distinction. Why do you think your God places differing value on men according to their free/enslaved status?
IF: (No real idea how he'll respond to this, but if he tries something like, "Well, since there's no longer slavery in our world, it's no longer relevant", then it would be fairly easy to follow up)
PG: Yes, slavery has finally been abolished through much of the world, but let's look at a theoretical example: in the Sudan, a formerly free man named Manute was recently captured by the enemy and enslaved. So according to your God, the life of Manute is no longer as valuable as the life of a free man, is that correct?
IF: (Again, don't really know how he'll respond. I think this could be particularly effective, especially given how strongly Americans believe in "equality of all men", and how strongly most Americans hate the idea of slavery.