hmm.. thats interesting, Tirmidhi mentions in this narration that Imam Ahmad acted upon it. perhaps there are multiple opinions attributed to Imam Ahmad on this matter.

hmm.. perhaps this is relevant

Al-Hakam ibn Uyaynah said that he went along with some people to Abdullah ibn Ukaym, a man of Juhaynah. al-Hakam said:

They entered and I sat at the door. Then they came out and told me that Abdullah ibn Ukaym had informed them that the Messenger of Allah (ﷺ) had written to Juhaynah one month before his death: Do not make use of the skin or sinew of an animal which died a natural death. Abu Dawud said: Al-Nadr b. Shumail said: The skin is called ihab when it is not tanned and when it is tanned, it not called ihab but na,es shann and qirbah (tanned skin or leather).

Grade : Sahih (Al-Albani) : Reference : Sunan Abi Dawud 4128

it also doesnt make sense to me that Ibn Abbas would continue acting upon it after the death of Nabi(s.a.w)

Ibn Wa'la al-Saba'i reported:

I asked 'Abdullah b. 'Abbas saying: We are the inhabitants of the western regions. The Magians come to us with skins full of water and fat. He said: Drink. I said to him: Is it your own opinion? Ibn Abbas said: I heard the Messenger of Allah (ﷺ) say: Tanning purifies it (the skin).

Reference : Sahih Muslim 366 d