You have 3 options for a one-time payment.
1) Add Earnings
You can add earnings to your team member.
If you want to add earnings, you can review this guide. How to add additional earnings to your payroll
2) Pay an Invoice
You can pay an invoice created by your team member.
If you want to pay an invoice, you can review this guide. How to pay an invoice?
Also, your team members can review this guide for creating an invoice. How to send an invoice to the company that you work for?
3) Pay a Reimbursement
You can pay the reimbursement request created by your team member.
If you want to pay a reimbursement, you can approve your team member's request in the Reimbursements section of the menu.