Iran Air was a mainly Boeing operator before the Islamic revolution and has considerable experience with the manufacturer from the B707 onwards. It continued to operate obsolete Boeing types due to sanctions.
When placing orders for aircraft, talking to more than one company will generally get you a better price even if you have already made up your mind which type to go for.
British Airways used to be so committed to Boeing that Airbus didn't bother trying for their business. When BA ordered A320s, their executives had to go to to Toulouse and persuade Airbus that they really were interested in ordering the type and not just playing them off to get a better deal from Boeing.