Fact #1: The IR-40 reactor did not need to be redesigned. Iran should've never accepted to redesign an almost complete reactor because of a concern (plutonium production) that could've been resolved through constant IAEA inspections.
Fact #2: China was under no obligation to help us redesign the IR-40 reactor. It was the US who was supposed to that in the JCPOA.
Fact #3: When the US pulled out of the JCPOA in 2018, China offered to redesign the IR-40 reactor to compensate for the US leaving the JCPOA. I don't know if China had issues over the price, but if the price was OK for the US, it should be OK for China as well, obviously.