According to TMZ Sports, a league representative said Johnny Football is still mulling the offer, while Hardy and former NFL tight end Kellen Winslow II have already committed to playing.
Manziel last played in the NFL in 2015 before getting waived by the Cleveland Browns due to a combination of mediocre performances and off-field issues.
The former Heisman Trophy winner struck a plea agreement in December that saw him avoid jail time after getting charged with misdemeanor assault against his ex-girlfriend.
When asked about an NFL comeback by TMZ Sports in January, Manziel said he was striving toward it and working out regularly.
Per TMZ Sports, Manziel will be subjected to nightly check-ins and a USADA drug test if he decides to play in The Spring League.
Hardy last played for the Dallas Cowboys in 2015 after five seasons and one Pro Bowl appearance with the Carolina Panthers.
He was charged with domestic violence against his former girlfriend, which resulted in him being placed on the commissioner's exempt list in 2014 before getting suspended for 10 games and then having that reduced to four games.
The charges against him were dismissed.
According to The Spring League's official website, it is a four-team league that will feature six games from from April 5 through April 26.
The teams will consist of veteran NFL free agents, and the deadline to apply to play is March 15.