Paid for eyegles only to find out that they were the wrong prescription. Who should pay for my new gles

On July 2007 I got a new pair of gles to improve my left eye vision. I had some trouble adjusting to the magnification prescribed by my doctor, but I kept using it, as I thought this would improve with time. On December 2007, I noticed that if I put the right lens of my gles over my left eye, I could see perfectly clear! I thought this was an error made by the optician, so I went back. To my surprise, he stated that the gles were done exactly as ordered by the ophthalmologist. So I went back to the eye doc, who checked my eyes again, and changed the prescription saying that I needed less magnification than what was previously prescribed.(the doctor did state that the gles were made to the specifications made on that previous prescription). Now the problem is that I spend alot of money on my glasses, and now the optician is asking me full price for the new lenses with the new prescription. My question is, what is the best way to go about rectifying this situation