Brown posted the letter to his Instagram story on Wednesday. The wideout's response read: "When your own team want to hate but there's no stopping me now devil is a lie. Everyone got to pay this year so we clear."
Raiders general manager Mike Mayock signed and sent the letter to Brown after an unexcused absence. Mayock informed the pass-catcher that he was being fined $13,950 for missing a team walk-through on Aug. 22.
The letter from Mayock mentioned the Raiders previously fined Brown $40,000 for missing training camp on Aug. 18, the same day the general manager issued an ultimatum to Brown.
Mayock also warned Brown that should he continue to miss mandatory team activities, the Raiders could impose additional fines and discipline.
The Raiders acquired Brown in a trade with the Pittsburgh Steelers in March. After participating in the team's off-season program, he began camp on the non-football injury list due to frostbite on both of his feet following an accident inside a cryogenic therapy chamber in July.
Brown also was upset with the NFL for banning his 10-year-old Schutt Air Advantage helmet, the only helmet model he has worn in his professional career. He lost two appeals to use the model.
Brown announced Wednesday he will wear the Xenith Shadow helmet model this season.
"I choose Xenith," Brown told reporters. "It's just, I only get one head and one neck. So I got to make sure when guys [are] trying to take my head off, I got the right lid to protect [me].
"NFL athletes have a choice in what helmet they wear. I choose Xenith. It's not easy to make a change like this so far into your career, but the Xenith Shadow was the only helmet that could fit my needs on the field."
Brown believes the helmet makes him feel more agile and gives him better visibility, two factors that he liked with the Schutt Air Advantage.
The seven-time Pro Bowl selection recorded 104 receptions for 1,297 yards and 15 touchdowns with the Steelers last season.