Hi there, Mec4. Your customer has full control over how much they want to pay. They can choose to pay a partial amount instead of the full invoice by simply clicking the Edit amount button.
Once your customer receives the invoice, they’ll need to click View and pay. In the upper left corner, they will see the total amount due, and right beside that is the Edit amount button. From there, have them change the figure and then click Pay.
Please see the attached screenshot below for reference:

Let us know in the comments if you have follow-up questions.