What agreement did Perhacs and McGregor make regarding Hinton's polygraph, and how did the deal change?

Perhacs and McGregor had a deal that if Hinton failed the polygraph, McGregory could use it to convict him. If Hinton passed, Perhacs could use it to prove Hinton's innocence and show that they had arrested the wrong man. McGregor revoked the deal and would not allow the polygraph to be used as evidence.