A wild card in the seven months of extended negotiations between the U.S. and Iran over its nuclear program is the continuing plunge in global oil prices and its impact on Tehran’s finances.

U.S. and European officials said they had a growing sense earlier this year that Tehran and its Supreme Leader Ayatollah Ali Khamenei believed they were weathering the West’s international sanctions — despite the halving of Iranian oil exports since 2012, largely driven by a European embargo.