If we have a payment amount specified on an event, there is no way to override the amount.
An example is for our summer camp, the fee is 350.00. People pay 50.00 deposit 2 months before full payment is due, leaving 300.00 as a balance (which is the amount for the full payment event). however, we can take late registrations after the deposit due date, and then they would owe 350.00 on full payment due date. However, they cannot use paypal for the full 350.00 due to the amount of 300.00 being the event payment.