Advance payments

[devmount]

Is your feature request related to a problem? Please describe.
Sometimes you want to get a fraction of the payment in advance (usually around 30 - 50%).

Describe the solution you'd like

• Invoices have two additional fields for advance payments
  • Amount (currency input)
  • Payment received date (datepicker, defaults to today if Amount is set the first time, can only be saved if Amount is set)
• Advance payments are taken into account for actual taxation calculations
  • Test for rounding errors when calculating final invoice amount
• Invoice documents contain a text snippet stating the already paid amount and the remaining amount to be paid with this invoice

Additional context
A client of mine wanted to pay in advance, so I became aware that I needed that feature. In general it is a good idea to utilize advance payments, especially for new clients.