To take a Quick Payment, go to People > Clients > select the Client > Accounting and click Quick payment. In the Take Quick Payment window, the Payment Type dropdown gives you two options:
Top up the client's account: Adds funds to the Client's account, which can be used for future Invoices. This generates a paid Credit Request.
Take a payment and generate an invoice: Creates and immediately pays an Ad Hoc Charge. This is ideal for one-off fees such as registration costs or resources. It does not affect the Client's balance. Choosing this option adds an Ad Hoc Charge Category dropdown so the charge lands in the right category for your reporting.
Quick Payment menu.
You'll also be prompted to enter an Amount and a Description, and you can tick Send PDF to email the Client a receipt.
If taking a card payment, you can choose to use a saved card or enter a new one. Once submitted, the relevant Invoice or Credit Request will be automatically recorded against the Client's balance.
Note: Quick Payments are processed through Stripe, so the Quick payment button is greyed out until your Stripe account is connected under System > Settings > Payment Integrations. See here for how to set Stripe up. The button also only appears for Administrators with accounting edit permissions.