There are two ways to take money before the work is done. Which one to use depends on whether your customer needs an invoice for the advance. For advances you bill regularly, such as a monthly retainer, see the third option at the end.
Only Administrators and Finance Managers can create invoices, credit notes and payments.
If your customer needs an invoice for the advance
The advance invoice gets paid as normal. A credit note then carries the money forward, so the invoices for the work can be settled from it.
Create an invoice for the advance as usual. Put something like "Advance payment" in the Subject, then send it.
When the money arrives, record it with Record Payment, or let your customer pay online. The advance invoice is now Paid.
Go to Invoices › Credit Notes and create a credit note for the same amount.
As you deliver the work, invoice it as usual. On each invoice, choose Actions › Apply Credit Note and allocate some of the credit, until the credit note has none left.
You can also start from the credit note: Apply to Invoice lets you spread the credit over several invoices.
The credit note's Available Credit shows how much of the advance is left. Once it's all used, the credit note is Closed.
If your customer pays without an invoice
Record the money as a payment on its own, then apply it to invoices later.
Go to Invoices › Payments and click Record Payment. Choose the customer and click Create.
Enter the Payment Date, Payment Amount and any other details, then click Save Payment.
The payment stays Open, with the whole amount as its Available Balance.
When you invoice the work, open the invoice and choose Actions › Apply Existing Payment.
Type the Amount to Allocate from the payment, then click Apply Payment.
Repeat steps 3 and 4 for each invoice until the payment's Available Balance reaches zero. Or open the payment and click Apply to Invoice to spread it over several invoices at once. See recording partial payments for more.
Either way, credit notes and payments can only be applied to invoices for the same customer, in the same currency.
For regular advance payments
If you bill in advance on a schedule, use retainer subscriptions. Avaza creates each retainer invoice for you, and once it's paid, the credit is drawn down on later invoices with Actions › Apply Retainer Credit. Retainer invoices need the Retainer Invoices module, which an Administrator or Finance Manager can switch on under Settings › General.