This one sounds a bit weird, but stay with me for a minute.
You know there are two different worlds on Earth. The world you an I live in, and the world accountants live in.
Two different realities. Not much in common.
Well, in the logical world, an invoice is an invoice. Wether the amounts are positive or negative doesn't matter. If it's positive, you owe me money, if it's negative… well, shit happens.
BUT… accountants have a long tradition of making things more complex. They say: if the total is negative, an invoice becomes a credit note, and thou shall print the opposite number of each amount.
All legislations everywhere allow negative invoices, but accountants are accountants, so making things simple is out of the question.
Now, in FileMaker, this has an important impact on the database structure. If you have a typical 2 table structure (header and lines), with taxes, discounts… not counting payments, deposits… Well, that is a lot of fields to duplicate! And you need another layout to print that.
One single feature could solve this. In the data inspector/number formatting, a checkbox to display the opposite number. Then the formatting options would apply normally, but after the number has been multiplied by -1.