Sold my iPhone on ebay and buyer requesting to return it?

Sold my iPhone on ebay and buyer requested to return it last week because of the screen being bent. ( Which it wasn't) or a partial refund of £40. I replied to his message telling him that I'd give him a £40 partial refund. It is now a week later and I've asked him to send me the phone back for a full refund but now he is telling me that he has already changed the screen and so he wants the full refund + £40 which he has paid to get a new screen.
I'm just wondering, can I now say that I will not refund him at all and will not accept a return because the item has now been tampered with?

Yep if he's fixed the problem then you don't have to refund him at all. He should've waited for you to refund it before he fixed it. Sounds like a scam to me.

Sounds like a scam, I hope you have a note of the serial number and photos of the phone, check his feedback, don't sell to anyone with less than (say) 50 feedbacks or below about 97%

He can't ask for a full refund unless he sends the item back. Sounds like a scam to me.