He doubled down on his claims that media consultant Laundry Service tried to extort $6 million from the pizza chain.
Laundry Service has yet to comment on Schnatter's accusations.
"He's not going quietly", Schnatter's lawyer, Patricia Glaser, told the Journal.
He also said in the letter to the board that the agency asked for a higher payment than had been agreed to.
Papa John's global founder John Schnatter told the pizza company's other board members on Saturday that he regretted stepping down as the company's chairman last week, saying he gave into pressure without a fair investigation the facts surrounding his admitted use of racial language on a private conference call in May.
Schnatter apologized for using the racial slur and announced his decision to step down last week. The board is now trying to kick him off entirely and, in response, Schnatter has now said it was a mistake for him to step down as chairman.
Schnatter is still a member of the board, but is no longer allowed to use the offices at Papa John's headquarters in Louisville, Ky., and will be removed from all advertising and marketing materials.
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There were also photos of the team holding and kissing the winning trophy in the rain. France's national football team players parade down Champs-Elysee avenue in Paris.
Papa John's also continues to cut ties with Schnatter over the incident.
Schnatter said the discussion only started when he objected to the agency's suggestion of hiring rapper Kanye West as brand spokesman because West "uses the "N" word in his lyrics".
During the call, Schnatter was said to be participating in a role-playing exercise, when he allegedly stated that "Colonel Sanders called black people n--", and claimed that Sanders never got backlash for it. Glaser told CNBC the board doesn't have authority to remove him without shareholder approval.
"Those individuals were acting on rumor and innuendo, without any investigation - let alone a third-party investigation of the facts", Schnatter wrote of the board's actions. The board chose to kick him out of his office at Papa John's headquarters in Louisville, Kentucky.
Schnatter reiterated his side of events in the letter, stating that Laundry Service was brought in to conduct a "diversity media training" session ahead of a conference he was attending.
- Add Morehouse College to the growing list of places parting ways with Papa John's. And I think, again I don't care if the setting was confidential or not, it hurt somebody's feelings, ' Schnatter said.