The City Reds were found guilty of breaching Rugby Football League rules when they appeared in front of an RFL operational rules tribunal in Leeds.
Salford were charged after Castleford pointed out that three plays took place while the Reds had an extra man on the pitch after making a substitution three minutes from the end their home match against the Tigers.
Prop forward Darrell Griffin, who was due to be substituted by the Reds, stayed on the field and made a tackle near his own line before going to the bench.
Salford held on for a 34-30 win in coach Brian Noble's first match that temporarily lifted them off the foot of the table.
The Tigers lodged a formal complaint and, after a three-hour hearing and deliberation, Salford were deemed to be in breach of three operational rules.
Salford faced the risk of being docked competition points but a precedent was set in 2006, when St Helens were charged with misconduct after a similar incident against Hull and subsequently fined £100.
In Australia in 2009 Canterbury Bulldogs forfeited winning points against Penrith for fielding 14 players during their 28-26 victory.
Salford are currently bottom of Super League, although only on points difference, with three wins from their first 14 matches.
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