We have a field which is decimal (9,2) and another which is decimal
(9,3). Is there anyway to subtract the two and get a precision 3
value without changing the first field to 9,3?

For instance, retail value is 9,2, but our costs are at 9,3 due to
being averaged. To calculate margin (retail-cost), we want that also
to be 9,3, but a basic subtraction comes out 9,2. You can see we
don't want to increase retail to be 9,3 (that would look funny), and
it seems wasteful to store retail twice (one 9,2 for users and one 9,3
for margin calc)...is there any other way?