The 20-year-old midfielder completed his move to the League One side this week with the Ibrox outfit claiming to have retained a "signifcant future sell-on percentage as well as return options and matching rights on Charlie in the future".
Rangers insisted the transfer fee would remain "undisclosed", but the amount has been reported to be in the region of £350,000 for a player who made eight first-team appearances for the Glasgow side.
That figure has been disputed by Forest Green chairman Dale Vince, who insists his club paid nowhere near the amount quoted for the Northern Ireland international and former Manchester United youth.
Vince, who this week appointed former Rangers and Everton striker Duncan Ferguson as the club's new manager, said: "That figure is what Rangers said. It isn’t true. It’s a fraction of that price."