Hi Christian,
The problem is that we don't necessarily know what invoice you deleted. Say for example you created invoice 1,2,3,4 and then decided to delete invoice 2. Since the next number is 5, you would not want to jump back to 2 because after that you would end up having two invoice 3's and two invoice 4's. If you must revert a number it is better that you manually do it so you know what is happening.
Also, you probably should not be deleting invoices. You can easily change information on an invoice. You can add or remove Time or Expense entries from an invoice as long as the Status is Pending. If you just need to change information on a Time or Expense entry, if the Status is Pending, you can change that too and the invoice will reflect the changes.
Hope that helps.
