By the word "Credit", I believe what you mean is a "Bill Credit" and not "Credit Memo".
And I believe, in the Bill Credit you are using the same item as you use in Vendor Bill, right?
If my assumptions are correct, then the problem is you are using the same item as in the Vendor Bill, you should use a different item which the expense account is directed to "Gain or Loss on Asset Disposal" account.
If you do this, the GL Impact of your Bill Credit will be
(Dr) Account Payable
(Cr) Gain or Loss on Asset Disposal
Then, when you dispose your asset on Dec, the GL impact will be
(Dr) Accumulated Depreciation
(Cr) Asset
(Dr) Gain or Loss on Asset Disposal