You can use a Payment Schedule to specify how and when a client pays you. Although each payment in a payment schedule specifies how much is due (and when), you can optionally allow clients to make partial payments or overpayments on invoices.
When your client is paying for an invoice that used a payment schedule with partial payments and overpayments turned on, they'll be able to specify how much they'd like to pay instead of assuming they're paying the full amount.
If your client pays less than the invoice amount, the invoice will be marked as "Partially Paid", and if they pay more than the invoice amount, the overage will be applied to the next invoice. They will only be able to pay up to the amount of the total outstanding balance for the order.
How to enable partial payments and overpayments
To enable partial payments and overpayments, enable the "Partial Payments & Overpayments" toggle while editing/creating a payment schedule.
When you have partial payments and overpayments enabled, you can optionally disable or enable partial payments and overpayments for deposits/retainers. If you'd like to allow partial payments and overpayments for all payments except the deposit/retainer, keep the "Allow for Retainers/Deposits" toggled off.