Guitar Purchase Dispute Resolution

I am receiving guitar lessons at the same facility I bought a guitar. Turns out they sold me a right hand guitar when I need a left handed guitar. The teacher I am taking lessons from has a good relationship with yhe guitar center since its in the same building, so we ended up doing a fair trade. I got a guitar that fit my need for left hand playing. My teacher, the sales person and I were all present when the sales person called the owner to see if it was okay to switch the guitars. The sales person described the guitar and gave him the guitar # to the owner. The owner agreed that it was a fair trade and we switched the guitars. I have now had the guitar for * weeks and went in for another lesson. I arrived and my teacher told me now the owner is saying he made a mistake and that the guitar is $* more. Do I have to pay the extra *, or is it the owners fault and I should be able to keep the guitar since it was already given to me.