Billy Long, a former Republican congressman from Missouri whom President Trump has tapped to lead the Internal Revenue Service, encouraged people to claim a tax credit that the I.R.S. has said does not exist, according to the company that offered the tax break.

Mr. Long’s effort to promote the tax credit, along with his peddling of a separate, fraud-ridden pandemic-era tax break, will be under close scrutiny on Tuesday when he appears before the Senate Finance Committee for his confirmation hearing.

After leaving Congress in 2023, Mr. Long, who had no background in tax, began working with a web of entities that made questionable promises to taxpayers of large I.R.S. refunds, according to his financial disclosure and previous reporting by The New York Times.

One of those entities was White River Energy Corporation, an Arkansas-based oil and gas company. The firm has said it joined with an unnamed tribal entity to sell “tribal tax credits” to people who wanted to claim the supposed credits and reduce their tax bill.

In a statement last month, the company said Mr. Long “made an insignificant amount of referrals of these credits to third parties.” White River also defended the credit and said that the federal government had never told the company to stop using it. Bloomberg Tax first revealed White River’s business practices.

A financial disclosure that Mr. Long submitted as part of his confirmation process shows him receiving at least $5,000 in compensation from White River, on top of tens of thousands of dollars in payments for work with other companies that encouraged clients to file for large refunds, including by using the supposed tribal credit.