If we were talking about an off-the-shelf product that China has finished developing, sure, China would be able to sell it to who it wants and no-one would be able to say or do anything about it.
The issue here is that China doesn't want to fund development of the FC31. That means if SA (or Iran) buys into the project with the funds needed to take this project to completion, they will be buying more than just airframes, but also a share of the rights to the IP of the plane itself. With those rights, comes powers, chief of which would be a veto on who cannot get it.
I also do not see why China would want to play god in the ME to try and balance everyone against each other. That is also surely an impossible dream because even today, the balance of power between Iran and SA are overwhelmingly biased in favour of SA (at least on paper). If China really was obsessed with creating balance in the ME, it should have started pumping arms to Iran years ago rather than selling to SA and their Gulf state buddies.
China has thus far tried to keep its dealings with ME countries largely commercial and unbiased (even with SA).
China tries not to pick sides or even favourites, and it has been working fairly well for them thus far, so it would be very odd for them to suddenly tilt so much in favour of Iran.
One of the biggest selling points of the FC31 would be the chance to join the project on the ground level, and get a say in the project direction