So who called you to say that you had to now pay in sterling? Was it the sales man that took the initial deposit from you. Did he make a mistake? Or was it another sales man.
Legally speaking ( and i wouldn't take this as being right ) since there is not currency symbols on the invoice and nothing relating to a special offer, there might not be anything you can do. It might come down to your word against them.
You have 2 options that i can think of:-
1) you pay the remaining amount before saturday, and get your sofa, put it down that there robbing lying gits and never use them again!
2) wheather you have the money or not..... you refuse to pay the remaining balance untill the sofa is delivered, ( if they deliver it ) and then just pay them in euro and see what happens, or pay them the remaining balance at the steriling value.
Personally if it was me i would kick up a fuss, i hate it when companies do this kind of thing, tell you one thing and then go against what they said. Kicking up a fuss might be enough for them to drop the matter and honor their orginal agreement.
I would also want a damm good explanation as to why they have changed what they orginally said.
And then in saying that it would also come down to how much i liked the sofa.