Enter layout mode.
Use the text tool to add TBD to your layout.
Give it this conditional format expression:
YourTable::YourAmountField > 0
specify a font size change for this layout text to make it about 120 points or larger.
Select your amount field.
YourTable::YourAmountField = 0
to set the contents of this field to 120 points or larger in size.
postiion the TBD text on top of your amount field.
Note, the large font size will make the layout text and the contents of your amount field disappear if you set a large enough font size 120+ points is usually sufficient to produce that result.