Selling Iran’s oil in defiance of an American embargo was already a risky and complicated venture for Iranian brokers. Then came the precipitous plunge in prices because of the global coronavirus clampdown, which has shriveled demand.

At first, Iranian brokers and many others in the country watched with amusement as U.S. oil prices briefly plunged below zero on Monday — meaning sellers had to pay buyers to take the oil away. Then on Tuesday the price of Brent crude oil, a benchmark blend that more directly affects Iran’s price, dropped to $20 a barrel, the lowest level in nearly two decades.

By Wednesday, Iranian oil brokers, who already sell at a discount to evade American sanctions, were nervously calibrating how far prices could drop before they would lose money.

The oil market’s collapse this week was another unanticipated blow for Iran, where the authorities have struggled to contain the worst coronavirus outbreak in the Middle East while keeping afloat an economy that has long relied on oil exports but has been hampered by American sanctions.