Flynn found it hard to retain a regular place in the starting line up after the introduction of Australian winger Corey Thompson at the start of the 2016 season.
Flynn was loaned to Castleford Tigers in May to give him the opportunity of first-team football, where he made ten appearances and scored six tries for the Yorkshire club.
Sa has decided that he is to head back home after just one season at the club.
Sa joined the Vikings from Hull FC for the start of 2016, and went on to make 13 appearances for the club in an injury-hit season.
Also 20-year old Almer Salvilla has asked to be released from his current contract after deciding to leave rugby league and pursue other potential career opportunities.
Salvilla has represented the club for a number of years, and was awarded the Under 19’s Player of the Year award in 2014.