'Never in my worst nightmares': Taylor Swift on Justin Bieber's manager buying her back catalog

Scooter Braun, Kanye West's former manager and Justin Bieber's current manager, has agreed to pay more than $300 million for Taylor Swift's back catalog -- all of her songs from her 2006 self-titled debut album up through 2017's "Reputation."

Ithaca Holdings, owned by Braun, is buying Big Machine Label Group, Swift's former record company, owned by Scott Borchetta, according to Billboard.

"When I left my masters in Scott's hands, I made peace with the fact that eventually he would sell them," Swift wrote on Tumblr. "Never in my worst nightmares did I imagine the buyer would be Scooter. Any time Scott Borchetta has heard the words 'Scooter Braun' escape my lips, it was when I was either crying or trying not to. He knew what he was doing; they both did."

Swift left Big Machine in November to join Universal Music Group, which is set to release her latest record on Aug. 23.

Swift said she learned about the deal "as it was announced to the world," but a source told Billboard her camp first learned of the deal on June 25. ABC News has not independently confirmed when those closest to Swift were told a deal was imminent.

Her father, Scott Swift, according to Billboard, owns a 4% stake in Big Machine, and a lawyer for her management firm called into a meeting to discuss the sale. Braun intends to buy out all minority shareholders, including Swift's father.

Swift has accused Braun of being involved in the infamous Kim Kardasian leak of a phone call that attacked Swift's honesty. Swift also claimed Braun has encouraged West and Bieber to bully her online. Additionally, in West's video for "Famous," one of the lifelike mannequins lying naked on a bed looks shockingly like Swift. She blames Braun for this, calling it "revenge porn."

"Now Scooter has stripped me of my life's work, that I wasn't given an opportunity to buy," Swift wrote. "Essentially, my musical legacy is about to lie in the hands of someone who tried to dismantle it."

Swift also ripped Borchetta in her post, which she signed off by writing, "Sad and grossed out, Taylor."

Borchetta responded in a post on his label's website. He attempted to refute her accusations point by point, providing what appears to be copies of texts and contracts supporting his argument. He also said Swift's father, her lawyer and one of her managers were on the shareholder call five days before a deal was announced.

Additionally, Borchetta provided what appears to be a screenshot of a contractual offer to Swift to stay with Big Machine that included: "100% of all Taylor Swift assets were to be transferred to her immediately upon signing the new agreement."

"Taylor had every chance in the world to own not just her master recordings, but every video, photograph, everything associated to her career. She chose to leave," Borchetta said.

Bieber, via Instagram, said Swift's post slamming his manager was "crossing a line."

Referring to Braun, in discussing claims he was asked to bully Swift, Bieber wrote: "He was the person who told me not to joke like that...Scooter has had your back since the days you graciously let me open up for you!"

Bieber said it "isn't fair" to attack Braun on social media and "get people to hate on" him.

"What were you trying to accomplish by posting that blog?" he wrote. "Seems to me like it was to get sympathy u also knew that in posting that your fans would go and bully scooter."

As of late Sunday, Swift had not responded publicly.