partial invoice payment

We would like the ability to generate an invoice for a member that is a specific amount less than the total due.

In other words, so the member can make partial payment against a single fee. We invoice for a full session at once, and individual event fees can be over $1000. For members who request a payment plan, it would be so helpful if we could generate an invoice for them for our agreed-upon-amount (which is an amount less than the total fee due), and the member could click into the invoice to pay.  If the fee is $1,000, for example, we'd like to be able to generate an invoice for $500 for that customer to pay now, then the remaining $500 later.