The Oakland Raiders WR posted a photo of the disciplinary letter he got from Raiders GM Mike Mayock, explaining why he was docked $13,950 for missing a walk through on August 22 and $40,000 for missing training camp on August 18.

Brown was MIA on both of those dates due to issues with his helmet -- it's been well-documented and was a main story line on "Hard Knocks."

Brown has since returned to practice and has a helmet he likes -- but clearly, he didn't expect to get hit with fines ... especially with head coach Jon Gruden defending him publicly.

"When your own team want to hate but there's no stopping me now," Brown said on social media.

"Devil is a lie. Everyone got to pay this year so we clear," he added.

FYI, August 18 is the day Mayock called out Brown to the media -- saying he either needs to be "all in or all out."

August 22 was a preseason game and, while AB was in attendance, the Raiders claim he blew off the pregame walk through.

The NFL regular season kicks off this week -- the Raiders play the Broncos on Monday Night Football -- and ya gotta wonder how this new drama will affect him on the field.

Here's a transcript of the letter Maytock sent to Brown informing him of the fines.

"As you know, you did not participate in the Raiders' walk through on August 22. Your absence from practice was unexcused. Accordingly, you are hereby fined $13,950 pursuant to Article 42, Section 1 (a)(viii) of the NFL Collective Bargaining Agreement and the Raiders' Club Discipline Schedule. 

You were previously fined $40,000 for missing Raiders' preseason training camp on August 18. Please be advised that should you continue to miss mandatory team activities including practices and g ames, the Raiders reserve the right to impose additional remedies available under the Club's Discipline Schedule, the CBS and your NFL Player Contract, including but not limited to, additional fines and discipline for engaging in Conduct Detriminetal to the Club.