If you look at the history of the pepper coast, grain coast... and where slaves come from... you will find the bulk of the slaves came from what used to be called the grain coast or the pepper coast wheich refers to basically the same area. Then look to who the discoverers were, who the traders were and you will find portugese all over it. Trading in gold then slaves as per wikipedia.
Accounts of that area were that they seemed largely uninhabited. Matter of fact, Liberia was chosen for the Monroe slaves in the early 1800s as it was relatively unoccupied.
I do know that if you go inland in Sierra Leone, Liberia, Ghana, Togo, Cote de Ivoire in the backsides there are places that are still today relatively uninhabited. AND Voodooo is still king of the bush. Not islam. Although Christianity seems to have a stronger foothold than Islam.
Back two hundred let alone three, four five hundred years ago islam wasnt even on the map...Voodooo was absolute king.
Mali is its own story and very few slaves came from there. The vast majority came directly off the coastal areas through the Potugese infrastructure.
As you indicate, the Portuguese were fanatically Christian, which suggests that they would have a low probability to import Islam
The people carrying human beings across the Atlantic to be sold as slaves in the Americas probably didn't care what religious beliefs their captives had, whether animistic, Islamic, or Christian. I don't think there were many Muslims brought to what is now the US, but I think there is evidence for a least a few. In any case that doesn't mean that "Muslims built America"--a few of them may have added slightly to the wealth of a few plantation owners, who probably spent it on imports from England.
There was a notable slave uprising in South Carolina about 1739 or so. I read an article once which argued that the slaves involved were Christians from the Catholic kingdom of Kongo (near the mouth of the Congo River), who may have known that the Spanish in Florida were Catholic and were trying to escape to Florida. The rulers of Kongo had been converted by the Portuguese.