https://www.cbssports.com/nfl/news/agents-take-how-key-players-opting-out-affects-salary-cap-for-2020-and-beyond-plus-more-opt-out-details/
"A player voluntarily opting out will receive a $150,000 stipend provided he earned a credited season (three or more regular season or postseason games on the 53-man roster, injured reserve or physically unable to perform list) in 2019 or was selected in the 2020 NFL Draft. Undrafted rookies can opt out but won't receive a stipend. Players opting out voluntarily won't receive a credited season or accrued season (i.e., a year of service for free agency) for 2020.
The $150,000 stipend is considered a salary advance or loan that will be offset against any money earned in the future from playing in the NFL. Technically, a player would owe his team $150,000 if he never played another down of football after opting out. The lengths a team would go to collect the $150,000 remain to be seen."