Partial refunds cannot be enforced by eBay, this is solely to be agreed by both buyer and seller, and it would seem the seller here, is not going ahead with their promise.
Are you still within 30 days of the item's last estimated date for delivery, then your only option is for a full refund, or to keep a broken item.
For a full refund, proceed this way.
You have received an item not as described, so you are guaranteed a full refund under eBay's 30 day MBG, Money Back Guarantee policy, if you claim within 30 days of the item's last estimated date for delivery.
You then go right of the item in your Purchase History, and from More Actions select Return this item, selecting the reason... Item Not as Described.
The seller has 3 days to accept the case, if they don't , you need to escalate on day 4, or within 30 days, and eBay will step in and force the refund.
If the seller wants the item back first, they need to pay for a fully tracked return postage. On receipt of their item they have 48 hours to refund you. Again, if they don't, you escalate the case.
Do not close the case early for any excuse or promise the seller may give, and do not forget to escalate.
