Putting money where your mouth is requires that you do put your mouth in the first place. If the money isn't forthcoming, the mouthing was meaningless.

Abolishing debt burdens on poor people is an obvious duty in itself, as an aspect of common humanity. Linking it to an obligation incurred through slavery, the suffering caused on the one hand and the profit unjustly acquired on the other, is justifiable in itself, and could also help in actually motivating the changes required.

Tying it to individuals on a basis of whether they could prove descent would be a blind alley. It would lead to nonsenses under which, for example, someone who was partially descended from slave-drivers and partly from slaves would be entitled to less than someone who could prove that all their ancestors at the relevant time were slaves.