Originally Posted by Tchiowa

If the receipt is the 390EU then you're covered. You can make them honor it. If it's the higher price you mentioned then you have no proof that UA accepted the lower fare.

Go back to the original site and look at "the fine print". Sometimes these low prices (and this is low but not spectacularly low) are "Each way, based on R/T purchase". So if you bought one way you didn't get it.