If you can't take them back you have several options.
1) Use them as they are with the M cartels, not ideal but perfectly doable.
2) Sell or swap your boots or bindings.
If the boots fit your feet well and you like them I would be inclined to change the bindings. 2012 cartels will sell easily and since they are on sale you could probably make the change for very little money.
Or try a swap maybe someone on here wants the M and has the L which is too big for them.