New details have emerged about the long-standing tension between Facebook and its biggest acquisition, WhatsApp, according to The Wall Street Journal.

The Journal reported on Tuesday that WhatsApp co-founders Brian Acton and Jan Koum gave up $900 million and $400 million, respectively, by leaving Facebook before they were eligible for stock awards. Their departure followed mounting pressure from Facebook executives to consider advertising and other monetization methods, the Journal said.

The pair, both vocal critics of digital advertising, had an escape hatch built into their contracts, the Journal said. The clause allowed for accelerated payouts in the event that Facebook insisted on making any “additional monetization initiatives,” but it was ultimately not used by Acton and Koum.

Petty clashes between WhatsApp and Facebook over chairs, bathroom designs and desk sizes eventually escalated, the Journal’s report said. Acton tweeted in March, after he left the company, that he was deleting his Facebook account, amid outcry around the data exchange between Facebook and Cambridge Analytica. That move angered Facebook executives, one of whom called it “low class.”

For more on the fallout between WhatsApp and Facebook, see the report at WSJ.com.