“What policy kept freed slaves in the US?”
Not sure about “policy”, but it’s what was done: releasing some 6 million slaves into a population of only 40 million or so settlers. A huge percentage of settlers who didn’t volunteer to be here.
There were Christian organizations which sent Africans back to Africa. They bought land in Africa for them, and created a nation called Liberia. The capital is Monrovia (named for James Monroe.) They created a form of govt for it that is based on our own. Liberia, like many African nations, is facing a lot of turmoil now, but it has been a more peaceful and prosperous nation than most of Africa. It has an interesting history, there are videos on YouTube that you might enjoy. I am dredging up what I learned in high school over 50 years ago, and I am quite fuzzy on the details so I hope the few facts I wrote down here, are not misremembered. I do not think that any free black Americans were forced to go back, but they were given passage and financial help if they wanted to go. Some did, some didn’t. I think they did buy slaves freshly taken from Africa, and then took them to Liberia. However, as free men and women, and then gave them help in Liberia to become productive citizens.