EXCLUSIVE: Gabrielle Union’s multi-million dollar lawsuit against BET over claims that the Viacom-owned net played fast and loose with her Being Mary Jane contract just got a rather big boost from the biggest Hollywood union. SAG-AFTRA sent a grievance letter to Viacom yesterday over BET’s treatment not just over BMJ star Union but the entire cast, I’ve learned.

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The October 13 letter to Viacom’s Labor Relations department charges the media giant’s subsidiary Breakdown Productions inappropriately tried to exercise contract options on Union, Richard Roundtree and other Being Mary Jane cast members. Already in production on Season 4, BET want to produce that cycle and Season 5 of the hit drama back-to-back – a move that sparked Union’s lawsuit of earlier this week.

Not so fast, says SAG-AFTRA now. In its correspondence, the guild asserts that Union and all of the other regular cast members are under no obligation to participate in any such 10-episode fifth season.

In fact, SAG-AFTRA says any further production with the current cast, including Union, will only occur after negotiations for a further season option, an increase in their actors’ rate and immediate compensation of the difference in that rate from BMJ Season 4 to Season 5 is paid out. If Viacom’s BET and Breakdown Productions don’t agree, then the union says the next stage is the labor action of a formal arbitration – which is a whole thing unto itself, to put it mildly.

SAG-AFTRA declined to comment on the correspondence but confirmed that the union has taken action on behalf of the Being Mary Jane performers. BET had no comment on the correspondence from the actors’ guild. On the flipside, sources tell Deadline that Union’s suit was always intended not only for her but also for the benefit of her fellow Being Mary Jane cast members.

Union claimed in her $3 million breach of contract complaint that she only agreed to a 10-episode each Season 4 and Season 5 if she was given a break of around three months between the two cycles. In the October 12 filing in L.A. Superior Court, the Birth Of A Nation actor said that BET agreed to those terms and then suddenly pulled a fast one and slashed the break.