Blood and Magic
In this campaign setting, muls are not slaves, there’s elementals for that. They are simply the product of the union between dwarves and humans. Although, it should be pointed out that dwarves have been made slaves in the past so many times, that many dwarves and muls alike still feel sleighted against from a historical stand point. With the advent of the muls, half-elves, and genasi, most philosophers assume that humans can breed with nearly any other race, and will.