Do NOT close the case until you have the replacement in your hands.
If it has been more than 20 days since the replacement was 'sent' (If it has been) go to the Resolution Centre and escalate your dispute to a claim. Get your money back.
If you close the case, you will not be able to reopen it.
An impatient seller may only be impatient, but could be trying to scam you.
Tell him the item has not yet arrived, so you cannot close the case.
If by some chance, the replacement does turn up (it does occasionally happen) you can repay the refund using Paypal's Send Money option.