Prior to the Taliban takeover, Afghanistan, especially the city of Herat, was an important source of US dollars for Iran’s economy.
According to the Tehran based news site Moniban news, Iranian traders used to take Iranian made goods to Herat for sale; payments by Afghans were usually in cash. The rate of USD in Herat, which is only 2 hours away from Iran’s border, was lower than in Iran. So while in Herat, the traders purchased USD and brought them to Iran for sale.
Annually, the export of Iranian goods to Afghanistan was worth between $7 billion – $9 billion. With international sanctions on the way against the Taliban regime, Iran–Afghanistan trade could fall significantly. It could also reduce the supply of USD in Iran’s currency market, thus pushing up the rate of USD/Toman. This will then push up the cost of imports for Iran and its rate of inflation.
The departure of the US from Afghanistan is good for Iran’s anti-US rhetoric, but likely to be bad news for Iran’s economy.