Yes, the Spanish brought African slaves to Mexico, but the point that I was making was that even in the case of the African slaves, once they had been bought by a Mexican, they were essentially working under the conditions of indentured servants. They were treated terribly (but so were indentured servants, even in the US), however, they had certain rights, such as the right to marry, that true chattel slaves (in the US or even in other parts of Latin America) did not have.
Some indentures were a way of paying off debts, and were a contract with a limited term. Indentured servitude is a form of slavery.
In parts of the antebellum cotton/tobacco belt, freedom could be purchased, and slaves might be freed at death. It was still slavery.
But that isn't the usual way of doing things in slaveholding societies.
The Spanish imposed slavery throughout the Americas, the Spanish crown knew about it and profited from it, and the Spanish ecclesiastics swept out the Precolumbian cults, burning native codices as they went.