Elon Musk said he had confronted fellow billionaire Bill Gates over the Microsoft founder’s supposed holding of some $500 million in Tesla shorts in a series of testy text messages in which Musk rebuffed Gates’ request to discuss climate change philanthropy.

In an April 22 tweet, Musk confirmed the veracity of a text message exchange between him and Gates, in which the Microsoft founder admitted he was short-selling shares of Tesla, meaning that he was betting on the stock dropping in price.

“Do you still have a half billion dollar short position against Tesla?” Musk wrote in one of the messages, a screengrab of which was shared by the @WholeMarsBlog account on Twitter.

“Sorry to say I haven’t closed it out,” Gates reportedly replied, following it up with a request “to discuss philanthropy possibilities.”

Musk replied by saying he couldn’t take Gates’ proposal seriously if the latter was betting on a drop in Tesla shares.

“Sorry, but I cannot take your philanthropy on climate change seriously when you have a massive short position against Tesla, the company doing the most to solve climate change,” Musk said.

The @WholeMarsBlog account posted a follow-up tweet to the screengrabs, tagging Musk and asking him whether the exchange was real.

Musk replied by confirming its accuracy, though he said he hadn’t leaked it and it must have gotten out “through friends of friends.”

“I heard from multiple people at TED that Gates still had half billion short against Tesla, which is why I asked him, so it’s not exactly top secret,” Musk wrote.