JD Sports return issue

kfletch88

Hi all, hope you can help.

So I sent  pair of trainers back to JD Sports on 18th July via their free returns process which was an inpost locker and a returns label provided by the retailer, which was received back by them on 26th July but didn't receive any communication from them or a refund.
So I contacted them yesterday to find out what was happening, to which they have responded saying they cannot issue the refund as the item them received back in the parcel was an old pair of slippers and not the new unworn trainers I sent! Where do I stand with this as I am £100 out of pocket and they just keep saying that I didn't send the right item back. I 100% did and feel I've been scammed either by JD or the courier, but how do I prove this? I took a photo of the trainers as I was packaging them back up, but this obviously doesn't prove that that's what was in the package when I posted it. I have raised a dispute via PayPal however they have sent this to the retailer to respond, who are obviously just going to say the same to them as they are to me.
Any advice would be greatly appreciated.
Thanks.