Invoices and payments

I would like to be able to create an invoice against a sale that includes only a partial payment. The invoices only let me invoice an entire product.

For instance. I have a product sale of $500.

I agree to have customer pay $200 now and make payments of $100 each month thereafter till paid. I want sales person to follow-up with new invoices as well.

It would also be nice to have a payments section for invoices/sales.

Thanks. William