How can we improve WHMCS?

Hi,

I would like to request a feature that's available on other invoicing systems that I saw and which is very interesting. I would like to receive multiple payments for the same invoice for cases where my customer has multiple paypal accounts or doesn't have enough funding on one account. This way the customer could inform the value of the payment, make the payment and that would be credited to that invoice, having an outstanding balance which then could be paid with another paypal account or a credit card for example. This seems to be simple enough, by allowing the customer to inform the payment amount on payments and by filling automatically with the outstanding balance on the invoice. For example:

If there's an invoice of $100 for a customer, he can access his invoice and he's able to change the value (default to the outstanding balance of $100) of the payment to $20 and make the payment using his Paypal account #1. When he access his invoice again, his outstanding balance would be $80 and the value for the payment would be filled $80. The customer could then make a second payment using any payment method and setting a new value for the payment, which would also be credited to that invoice up to the limit of the outstanding balance.