You should be able to type something such as -249.95 with no problem. Do not
type any other characters or spaces. If the cell will not accept the entry it
probably has a Validation Rule applied which prevents the entry of negative
values. My guess is that someone else created the workbook file and they may
be the one who needs to make the change if necessary.
I created the workbook and I have not created any such rule for it. I have
managed to now put in a negative amount by right clicking and formatting the
cell but it still will not add this in the total column.