According to Shefter, Manziel would have been allocated to San Antonio, but the Commanders waived his rights, so the controversial quarterback will go through the league's waiver system.
Manziel signed the AAF standard player agreement on Saturday, according to ESPN's Adam Schefter, which will officially allow teams to sign the quarterback.
"We are pleased to welcome Johnny Manziel to Alliance of American Football, which we've always described as a league of opportunity for talented players to launch or revitalize their pro football careers", league co-founder Bill Polian said in a statement Saturday night.
He played just two seasons with the team before he was released.
The 2012 Heisman Trophy victor at Texas A&M had his contract with the CFL terminated this offseason.
The AAF, now in its first season, says Manziel will report Sunday to Memphis after the Express lost to Salt Lake, 22-9, on Saturday to fall to 1-5 on the year.
Information from The Associated Press was used in this report. The Express lost to the Salt Lake Stallions 22-9 in Week 6, losing quarterback Zach Mettenberger to an apparent ankle injury on their first offensive play. The Express next plays vs. Birmingham on March 24 in Memphis. He went on to be selected in the first round of the NFL Draft by the Browns in 2014. He was traded to the Alouettes in 2018 and released on February 27.
The league's due diligence included an assessment by an independent expert on domestic violence and a head-to-head meeting between Manziel and Ambrosie.
Manziel eventually ended up signing with Hamilton in May 2018 but was unable to get on to the field behind starter Jeremiah Masoli.
Manziel was released by the Montreal Alouettes of the Canadian Football League earlier this month.