A couple of things I would try...

Break the troublesome conditional into pieces. Try just one of those sums in your iif statement, make sure your syntax is correctly refering to the object you want.
Maybe even go back further to the basics and bring up the debug window and type in
msgbox ([qry_then_INBD]![SumOfAMT])
hit enter
and see if you get a legitimate value for each part.

Also try the Nz function. This will handle null values for you, by turning them into zeros (or anything you choose). something like Nz([qry_then_INBD]![SumOfAMT])
Lookup Nz in help.