like previously stated, unless you have a witness able to privide a stat declaration apon your verbal agreeemtn you have no chance. My reccomendaton would be to go to them, agree it was a misscomunication and find out if you can meet them half way or something with costing for the correct part installation. In my opinion this is all you will get out of this situation.
Any for reference, there has been a big discussion on legal fees ect. You dont need that for around $1000 disputes, apply to the consumer, trade and tenancy tribunal and fight it out yourself. Its private and mediated, not as duanting as some may think and its only $40 from memory. Ive been to plenty in the past for rental disputes and have never felt uncomfortable.