Hi, my account is in credit and the cost to leave my contract early is about $850.
I'm wondering if the credit can be used toward paying the $850 to cancel early (e.g. I'd pay ~$600 and the other $250 would already be paid with account credit).
Yes. Optus generally use any credit to pay each months bill before direct debitting etc. anyway. Any credit in your account after the final bill will also be posted out to you as a check (if applicable).
$850 is a high exit fee. If you'd like to post your circumstances (plan cost, phone, months into contract, lease?) we might be able to suggest something (but it is a bit of a long shot)
When the final Bill generates, it would automatically take up the remaining credits and give you the final amount. Everything should be detailed on your final bill. It should show how much credits you had before the bill was generated, and how much you were charged for that final bill ( with any early termination/cancellation fees and last usages from reset day to cancellation date) then the owing amount shoud be: How much you owe on the bill - credits = Total owing amount