“They should be happy with what we’ve already spent” does not follow from “Britian only finished off paying the bill for freeing the slaves to begin with” because “paying the bill for freeing the slaves” was paying the people who previously owned slaves compensation for them no longer having free chattel labor, not the people who were (and are descended from) the people who served as chattel labor.
or, to put it another way: none of the people who suffered, or who are descended from the people who suffered, have gotten anything “you’ve spent”―if they did, why would they be asking for reparations in the first place? they’d have in a sense already gotten them if what you were saying was true, and obviously they have not.