Complex sentence: He claimed to be interested in helping both races in his proposal, which was based on gradually freeing slaves after the age of 45 (when they would have repaid their owner's investment) and resettling them in Africa.
Split into simpler sentences: He claimed to be interested in helping both races in his proposal. His later views proposed gradually freeing slaves after the age of 45 (when they would have repaid their owner's investment) and resettling them in Africa.