I'd feel pretty comfindent that it did happen. But there is nothing wrong with it IMO. Just because China was at war with Japan pre Pearl Harbour (note: not pre ww2) doesn't mean the USA couldn't sell weapons and supplies to either nation.
Obviously Japan wouldn't be happy but back then the USA (and the allies) would have seen China as a more strategic ally due to the link with Russia and the war in Europe.
Before the USA entered WW2 many pilots flew for the RAF which is the same thing. It is also fact that some yanks fought for Germany which again isn't a bad thing, my own heritage is part German and without knowing how evil the Third Reich was I probably would have gone to fight for the fatherland. (If my mother wasn't English)
When it comes to suppling China with aircraft, one of the biggest events that caused the USA to join the Allies is because a Yankee ship got sunk by a German U-Boat because it was suspected that supplies were being transported.
I think the main thing here is that the USA views Pearl Harbour as an unwarrented attack on the USA but to be realistic Japan was being denied resources to fight China by America and unless they surrendered to China, there wasn't another choice. They needed the oil and other resources like rubber that indo china could supply. America and the British Commonwealth was aiding China by denying Japan those resources and Japan basically didn't take it kindly.
-Of course there is more to it but yeah...
