While the gesture is nice, and in theory it makes sense to do, the bureaucracy headache it would cause would be insane.
First you have to comb every record to find out who exactly is a descendant of slavery. And if someone is but they can't provide proof, then what?
Next you have to set a limit with their current income. What of they were a descendant of slavery but are now rich? Do they get money too? Is it fair for them to get something?
What about the poor who aren't from slavery? Are they just going to sit there and be all ... "cool" while their neighbor gets some money and they don't?
Native Americans, Latinos, people from the West Indies, Arabs, and certain European immigrants who may be poor and can't make it in a "white world" ... do they get anything?
The jealousy, anger, debate, et al this would cause is why it will never happen.