Apparently, Iran placed an order for 118 aircraft with Airbus in January, but that has to be approved by the US Treasury. There has to be a formal lifting of the sanctions.
Iran has complained about this, then decided that they could sweeten the deal by placing an order with Boeing, on the premise that if Boeing was also getting an order, Treasury would approve the deal faster.
I do not see how Boeing assumes the risk on this deal. Im sure there is a crony deal to stick us with the bill should Iran not pay up.