(1) The question is irrelevant, since President grant's purpose was not to use Santo Domingo as a base for the reintroduction of the African slave trade. In fact, one of his stated goals for the annexation was to undermine the Cuban slave trade with free labor.
(2) "Almost succeeded"? Really? His own Republican Foreign Relations Committee in the Senate refused to even put the idea up for consideration. Having a measure die in committee, in a committee completely controlled by one's own party no less, is almost succeeding? What then is the definition of failure?
That vote in committee by the way was 28-28. After that vote, a Commission was formed to further study the situation and recommended statehood for the Dominican, but public opinion was against it and Grant gave up.