The dialog that comes up shows the list of products that you defined earlier. You pick the one(s) that you want to sell to your client. At the bottom of the dialog you can choose whether you want to register the payment immediately e.g. in case the client pays on the spot. If you do not select this option, then the payment will remain pending and you can register the payment against the sale at a later moment.
If you had chosen to register the payment immediately, then you are done. If you do this at a later stage you would navigate to the Payments tab and register the payment there. The sales that have a payment pending are listed. If the client were to make an overpayment, then this would be added to their balance.
On the Payment tab in the client profile you can view the received payments by date, product and payment method. You can register a new received payment and select the sale to register the payment against. If you register an overpayment or a payment without sale, then this will be added to the client's balance.
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