Make a recurring payment with variable payment amounts

I need to make a form with recurring payments for a total amount, let's say $300, in 3 installations.

The users need to be able to say how much they want to pay for each recurring payment,  so one payment might be $100, another $175 and another $25 and they can choose the amount.

Is that possible?

I'm afraid that's not possible. The payments must all be equal. Please review this guide:

How-to-Setup-a-Recurring-Payment-Option