I would call and tell them what happened. At any LQS I go to they are reluctant to tell you exactly what you need and just for this reason. I wouldn't expect her to take it back, but I would let her know and see what she has to offer as compensation, if nothing, then you may not want to shop their again.
But I never put on a back without something that refelcts teh front, maybe a strip of color or a row of blocks, so I would just make the best of what you have if she doesn't offer you the remainder at a discount.