The Hull-based centre was revealed by a national newspaper to have bet £200 against Doncaster in their Challenge Cup fourth-round defeat by Wakefield at the Keepmoat Stadium in May 2011.
Leaf, who did not play in the game because of a thumb injury, won his bet after Wakefield scored four tries in the last 12 minutes to complete a 50-10 scoreline, the last of them passing the 38-point handicap they had been given on the betting coupon.
He was suspended for 18 months from May 2011 and fined £500 after being found guilty by a Rugby Football League tribunal of also betting against his team in a Championship One match against Toulouse in 2009 and of gambling on other games in 2011.
The 28-year-old physical education teacher recently began playing rugby union with Hull Ionians and has agreed terms with Doncaster for the 2013 season.
He took part in the Dons' pre-season training walk from Scarborough to Whitby last weekend and was present at the first training session of the new season on Wednesday night.
Doncaster head coach Tony Miller said: "I'm very pleased to have Shaun back at the club and he'll be a welcome addition to our squad.
"I'm looking forward to working with him again as he attempts to rebuild his rugby league career."