What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
We currently only offer the ability for one time payments or subscriptions with equal amounts for each period.
What you could do is set up a second form to pay a custom amount for the remainder of their balance. The Custom Amount or Donation Amount is available in the Paypal Payment tool.
You can see this option available during the configuration of the Paypal payment wizard.
Refer to the screenshot below
I suggest review also this user guide http://www.jotform.com/help/49-Order-Form-Types
And here is a jotform template http://www.jotform.com/form-templates/donation-form-2 that have a basic example of the Custom Amount/Donation using Paypal tool.