WASHINGTON — Lawyers for a detainee at Guantánamo Bay, Cuba, who is scheduled to be transferred out of the prison in December under a 2011 plea deal have asked a judge to order the Pentagon to live up to its agreement, recently unsealed tribunal papers show.

The circumstances of the unusual request are raising new legal quandaries about the tribunal court system. The possibility that the detainee may not be released threatens to undermine the ability of prosecutors to persuade other detainees to plead guilty — and to cooperate by serving as witnesses in more important cases — in exchange for a definitive date by which they would be allowed to leave the prison.

The dispute centers on Noor Uthman Muhammed, a Sudanese man who pleaded guilty to conspiracy and providing material support to terrorism in early 2011 before a military commission. Under the terms of his arrangement, he was to serve an additional 34 months on top of the nine years he had already been imprisoned as a wartime detainee, which would position him to be repatriated in December.

But Mr. Noor’s lawyers said that the Pentagon official who oversees the tribunal system, known as the convening authority, had yet to officially approve the final disposition of his case, even though it essentially wrapped up two years ago. They contend that the official’s approval is necessary to set in motion the time-consuming bureaucratic and diplomatic process of repatriating Mr. Noor.