Next applied amount and percent for retainers

The Next Applied Amount and Next Applied Percent fields functionality.

When the next applied fields are blank, the available amount of the retainer is applied against an invoice.
When a retainer is applied to an invoice, the expenses are sometimes less than the available retainer. The retainer applies up to the amount of the billable expenses. The remainder of the retainer is available to apply when invoices are generated with the apply retainer option selected.
When the next applied fields are specified, the amount or percent is used to apply to the invoice.
The next applied fields can be manually maintained if the amount or percent is changed between invoice generation.
The next applied fields can be manually put on hold or released between invoice generation.