So the problem from King Afonso's point of view was the industrial scale ? Like slaves being a tool and not a luxury or something like this ?
The problem was not so much the "status" of the slaves but the unprecedented scale of Portuguese slave-trading operations, which were far vaster than simply kidnapping some people from nearby villages, as had always happaned in Africa. Afonsos concern, in particular, was that, due to the size of their operations, the traders were becoming impossible to control. While it's true that most of the slaves were africans from rival states/tribe who were captured by the congolese themselves in their wars of expansion, Portuguese traders did not refrain from kidnapping congolese citizins, as well as people from their vassal tribes. Here's an excerpt from one of the letters Afonso wrote to king Manuel of Portugal:
"Each day the traders are kidnapping our people - children of this country, sons of our nobles and vassals, even people of our own family. This corruption and depravity are so widespread that our land is entirely depopulated. We need in this kingdom only priests and schoolteachers, and no merchandise, unless it is wine and flour for Mass. It is our wish that this Kingdom not be a place for the trade or transport of slaves."
Many of our subjects eagerly lust after Portuguese merchandise that your subjects have brought into our domains. To satisfy this inordinate appetite, they seize many of our black free subjects.... They sell them. After having taken these prisoners [to the coast] secretly or at night..... As soon as the captives are in the hands of white men they are branded with a red-hot iron.
Portugal did nothing to solve this issue, and slavery continued to be a source of dispute between Portugal and Kongo until the later was finally forced to completely submit to the former.
That's why I agree with Lusitania's assertion that both countries need to be in a relatively precarious position (Kongo defeated and Portugal stretched thing) so that they become forced to make significant concessions to each other. If this doesn't happen, there simply isn't sufficient alignment of interests for a true alliance to be maintained in the long run.