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.