I made a payment, but months later I realised that half of it wasn't paid to the freelancer, and it says Payment Remaining. How do I unlock this payment remaining to be able to get the invoice?
I checked the contract you are referring to, and it looks like the contract has ended already. You should be able to send a bonus payment for the freelancer by following the steps noted in this help article.
You can also go to your contract page, and beside the Rehire button, you will see the option to give a bonus for your freelancer.