Here are the NFL Waiver rules:
Players that have less than four years of experience are “waived” — meaning a player has to clear the NFL waiver wire before becoming a free agent. When a player is placed on waivers, the other 31 teams have a day to place a claim on him. The team with the highest spot in the waiver order is then awarded that player. If no team makes a claim, that player is then a free agent and can sign with any team.
Players with more than four years experience are considered vested veterans, they are not subject to the waiver process. As soon as his release paperwork clears, he is free to sign with another team.
Manning has 14 years experience so is immediately a free agent.