The conditions are simple but laughable. That the Aim-120s cannot be used in offensive operations against India, so that was a failure.
But the main point is the Pakistanis do not have access to the source codes that will allow its integration on any other jet. Only the Americans can integrate the Aim-120 on a Chinese jet, or vice versa, but it requires sharing of source codes by one country to the other. So you can see how impossible that really is. The Americans will reject it anyway.
And, even though it will fail, if China and Pakistan attempt to integrate the Aim-120 on a Chinese jet on their own, the Americans will slap sanctions on the PAF's F-16 fleet.
They can procure whatever they want from China, we can't stop it anyway. But we do not want Pakistan buying Western systems, we have some influence there.
The Chinese sell only export grade stuff to Pakistan, so it's sh1t anyway. Kinda like what the Soviet Union used to do. The Pakistanis are free to buy the HQ9 and waste more money, it's good for us.