There's a bit to unpack. That's why it's going through a higher court. I'm not following the case, nor am I up to date in QLD contract law or the laws governing agents, but the concept of someone misleading you into thinking you're signing something different, and someone being negligent in duty to you, is universal. Bear in mind there is more than one contract and more than one legal issue involved here.
If his agent can prove he was neither negligent nor misleading Payne, then the contracts should remain enforceable. If it's a matter of Payne not paying enough attention, not doing due diligence in reading the contract, not being aware of his market value or changing his mind, that's tough luck for him. Probably.
Best not to speculate and leave it to the courts.