WASHINGTON (Reuters) - U.S. House Natural Resources Committee Chairman Rob Bishop on Wednesday said Obama administration proposals to change a Puerto Rico debt relief bill are under negotiation, requiring the panel to cancel a Thursday work session on a Republican bill. “The administration is still negotiating on provisions of the legislation, creating uncertainty in both parties,” Bishop said in a prepared statement. He added that “it is unfair to all members to force a vote” in the committee as negotiations continue. Bishop, a Republican, had hoped to work on the legislation Thursday and hold a committee vote that would have sent it to the full House of Representatives for consideration. One congressional source told Reuters that the Thursday session was being canceled because the legislation lacked adequate support in the committee. 