Item sent in error.....who is responsible for return shipping cost

[iainhig]

I purchased a battery (from China), when it arrived the wrong battery had been sent.

I sent a picture the the seller who admitted they had sent the wrong battery and ask me to return it. They also emailed saying that they would pay for the return postage.

I sent the item back and have received a refund for the item but they are refusing to pay the return postage even though they agreed to it.

Can you advise on what, if anything i can do?

Thanks

Iain