Complex sentence: The free people of color developed formal arrangements for "placées", which the young women's mothers negotiated, often to include a kind of dowry or property transfer to the young women, freedom for them and their children, and education for the children.
Split into simpler sentences: The free people of color developed formal arrangements for "placées", which the young women's mothers negotiated. Under the system of "plaçage," often the mothers negotiated a kind of dowry or property transfer to their daughters, including freedom for them and their children, and education for the children.