sum has 35 distinct synonyms spread across 8 of its 8 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a quantity of money
“he borrowed a large sum”
a quantity obtained by the addition of a group of numbers
the final aggregate
“the sum of all our troubles did not equal the misery they suffered”
a set containing all and only the members of two or more given sets
“let C be the union of the sets A and B”
Synonyms, antonyms and the broader and narrower relations for sum come from Princeton University’s WordNet. Full sources and licences.