I'm a bit late on this, but I seriously doubt that most European powers' governments actually gave half a shit about the native Congolese, unfortunately.
Well, the Casement Report
had been rather thorough in denouncing the horrors of Leopold's administration of the Free State. The fact that enough people cared that a real propaganda war erupted between critics of the Free State and Leopold's personal propagandists also shows that there was genuine public outrage. The critics also cared enough to actually formalise their efforts via the Congo Reform Association... and their efforts were serious enough to eventually ensure their success: the Free State
was dissolved, and in no small part due to their efforts.
Perhaps governments as such cared far less, in fact that's quite plausible. But by 1908, the atrocities were well-known and widely reviled in Europe. Governments did care about public opinion (especially the opinion of "polite company", which by 1908 was quite revolted by the revelations of what Leopold had been up to).
In the end, I don't think a sale to belgium would be out of the question, but it would definitely look bad. If other options were available, those would certainly be preferred. And I'm just fairly sure those other options
would be available. I'm not even sure Belgium was interested in gaining extra colonies at that time...