Mario's advice sounds a good option, not having any facilities I would have to go for the exchange option but filing a small amount if it is a small amount could do the trick, it will depend on how much you think you will have to take off, use the 15t you have as a guide, I would be loathed to take out too much, so shaft to 15t sprocket gap is what you need.

It should work but if not you cannot then take them back, you could ask whoever sold you them to do the work, give them the gap measurements of the 15t sprocket as a guide.

Cheers,

Daniel.