The ending is not completely clear as to whether Fonny will be released.  There are signs in the ending that he will be free.  The reality that Baldwin constructs is one in which characters are able to persevere and eventually triumph.  In her review of the book, Joyce Carol Oates points to this fundamental optimism that is rooted in reality for Baldwin, but stark that there can be redemption in a setting where there is so much in way of pain and hurt:

Yet the novel is ultimately optimistic. It stresses the communal bond between members of an oppressed minority, especially between members of a family, which would probably not be experienced in happier times. As society disintegrates in a collective sense, smaller human unity will become more and more important...Fonny's real crime is "having his center inside him," but this is, ultimately, the means by which he survives. Others are less fortunate.

Oates' analysis might be accurate in terms of seeing freedom for Fonny.  While his trial date is delayed, what Langston Hughes might describe as "deferred," the reality is that justice and righteousness is on his side.  Baldwin makes it clear that the people around Fonny that fight for him, the "center inside him," as well as the basic idea that there can be hope and redemption in these situations helps one to believe that Fonny will be free.  The birth of his child will be into a world that will be conceivably better and more equitable, more hopeful, than the world in which his parents and grandparents live.  These sacrifices are what enables this child to have hope, to see his father be free, and in what Baldwin believes provides a chance a redemption when it is held in the balance with so many other forces around it.