You don't.  You want someone to type "1" but store -1?  Why?

Just adjust your calculations to always subtract.

If I indicate that I sold 1 widget, I would reduce my onhand inventory by
one.  I would not, however, store "-1" in my order details table.

If you do insist on doing this, I'd think you would do it using code in your
form.  Before updating the record just multiply by -1.

First the data type must be either single or double.  Open the table in
design view and click on the menu VIEW - Properties.
In the Default enter your negative value -    -1       or    -125

With an Update query, updating [fieldname] to

- [fieldname]

                 John W. Vinson[MVP]