Details forthcoming, but here's likely how it will play out.

Merchant uses gov't payment processor, merchant offers to customers to pay in bitcoin, but bitcoin payment goes to the government, merchant gets IRR.

The gov't of Iran can then either use the bitcoin to buy dollars, EUR, CNY, gold, whatever, or they could even use it for international purchases from vendors directly.

There's already a ton of commerce from Iran (individuals, companies, and government) in Dubai, Istanbul, etc, and bitcoin there are numerous bitcoin exchanges in those centers of commerce already, meaning there's no problem for Iran to convert the bitcoin into what they actual need or want.