[ShubhamS11]

What story or excuses you gave them at 1st can you tell?
Anyways, basically you have two options here from what i can understand
Either
Think of some ranting to rant at them on mail or phone call so that the Rep agrees to refund without taking the return, *they can do it, either by themselves or with their supervisor's approval. Your this^ story or excuse should be in alignment with the reason you gave them st very 1st when you started / initiated your this particular SE with them. Should be reasonably acceptable excuse AT THEIR END.
Or
Do you want to FTID/LIT thek for the return part? MY gut feeling If you opt for this, this method will fail surely IF THEY HAVE A PRACTICE OF ISSUING REFUND ONLY AFTER ITS MARKED RETURN RECEIVED FROM RETURNS DEPARTMENT.
Or send then Empty Box if they are responsible for loss of goods in transit its easy to go for returning them a Empty Box of that particular CPU with barcodes, sn, batch etc stickers on it(if you have those with you), they will assume that it was probably stolen by someone in transit.

I told them the cpu just stopped working sent them photos of computer all lit up and the monitor saying no signal simply just unplugged the hdmi cable I said I had another processor to test and it worked again told them I tested various components but it was obviously the cpu at fault.. After this he agreed for a refund..

I was planning on boxing but didn't know they would want to send someone around to collect it...

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@mothered  that's sounds a very good idea as I already told him I was late replying to his messages because I work away so this sounds the best option for me thanks guys

Do as mothered suggested you & you can then EB them. Sounds promising. Goodluck!