The game indicates that, in order to send resources to your allies, you need to advance to the feudal age and build a market. However, Abbasid Dynasty can send resources after advancing to the feudal age, without building a market.
I just tried and confirmed the bug. The correct behavior should be allow sending resource only if it has trade wing researched.
that is because trade wing can be used as a market
Well, when I tried it on a custom game with bots, I tried it with eco wing and then with military wing. Both of them allowed me to send recources. I saw this bug in a 2v2 game. My opponents were playing Abbasid and Zhu Xi. The abbasid player went eco wing and then sent all of his recources to his ally. In both my custom games, I chose my ally as Zhu Xi, too. So if you couldn’t send resouces as Abbasid on feudal except when you went trade wing, and if your ally was not Zhu Xi, then maybe the bug only happens in that specific instant. I can provide the replay if its needed.