North Korea billed the United States $2 million for hospital care for American student Otto Warmbier as part of an agreement to release him back to the U.S., and President Trump instructed the envoy that retrieved Warmbier to agree to pay it, The Washington Post reported Thursday.

According to two people familiar with the matter who spoke to the Post, the $2 million bill for the care of Warmbier, who was in a coma the entire time he was in North Korean custody, sat in the Treasury Department unpaid for about a year, and it is unclear if it was ever paid.

White House press secretary Sarah Huckabee Sanders told the Post that the White House would not comment on “hostage negotiations, which is why they have been so successful during this administration.”

The envoy that was sent to negotiate Warmbier’s release called then-Secretary of State Rex Tillerson after the North Koreans insisted they sign an agreement vowing that the U.S. would foot the $2 million bill. Tillerson called Trump and the envoy was instructed to sign the paper, according to the Post.

Warmbier was arrested in North Korea for allegedly pulling down a propaganda poster. He was placed on a public trial and sentenced to 15 years of hard labor. Warmbier was returned to the U.S. in a coma, and he died shortly afterward.

Trump has participated in two unsuccessful denuclearization summits with North Korean regime leader Kim Jong-un during his presidency. During the last summit in Vietnam, Trump said he believe Kim when he denied having knowledge of Warmbier’s treatment when he was in custody.