The majority of his wealth came from humans demand for salt. They found it could preserve food and it skyrocketed like microsoft did to the world. Everyone uses salt today.
He had indentured servants not plantation slaves.
Yup stuff like salt, gold, cotton and rice, etc., made the Mansas rich but the thing that made then extremely wealthy was the collection of taxes.
Mali literally sat on the crossroad of trade between the forest belt people in West Africa (modern day Guinea, Sierra Leone, Liberia, Ivory Coast, Ghana, Togo, Benin and Nigeria) and North Africa (modern day Morocco, Algeria and Tunisia). The Mansas literally taxed all trade caravans that came through their territory. Eventually Mali just became the midpoint of all trade that the Mande people controlled. That trade started long before the Mansas and Mali ever existed. It actually started during the Empire of ancient Ghana and likely even before then by another Mande people called the Soninke. They had multiple trade cities in the Sahel and one was called Tichitt Walata.
The Story of Africa| BBC World Service