The chairman of natural gas company Tellurian Inc. faces claims that he caused an investor to lose tens of millions of dollars by breaking a deal to reimburse him for holding stock as it plummeted in value.

In a complaint filed Thursday in the District of Colorado, shareholder Christopher Parker accused Tellurian founder and executive chairman Charif Souki of guaranteeing he’d hold his own shares and indemnify Parker for any losses incurred from diminishing share prices, then failed to follow through on both guarantees.