If you have completed the pre-agreed work and do not wish to deliver more work, you can raise an invoice for the pre-agreed work and amount.
If the Hirer declines the invoice, you need to raise a dispute. You need to read and agree with the Dispute Resolution Policy before raising a dispute.
However, if you wish to deliver the extra work that the Hirer is asking for, you need to ask for an extra deposit in the escrow account for that job. You can do that from the same Project workstream via Payments tab. Once you complete the work, you need to raise an invoice for the agreed amount to get paid.