This article is from CS monitor.
According to the article Iran has played an active role in Afghan reconstruction and stability in preventing the country from becoming a sanctuary for the Taliban. According to the article Iran has provided large amounts of funding making it one of the biggest donors for their war torn neighbor and has built roads, schools and commercial establishments.
I say, Iran's influence in Afghanistan is enormous.
This scenario is noteworthy to be cognizant of.
If sanctions are placed in Iran this will weaken Afghanistan's economy enormously, given the amount of trade between the nations. Iran can easily cut power and water in Afghanistan's big western cities of Herat and Farah. This will cause more insurgency (which has began growing of late) and major problems for coalition forces.