If a refund has been processed it's too late to stop it.
If the seller doesn't have the funds in the Available section of their payment summary, eBay will use their back up source which is usually the seller's bank account. When the refund is made, the payout funds are released. This really is their problem not yours.
Why are you agreeing to stop the refund? If the seller is promising a replacement, don't listen to them, just wait for your refund, you will receive it. It goes back to your original payment source, so if you paid with a credit card, it can take up to 30 days to appear back in that card account.
@syfleu_27