Mapping question

I am trying to map Salary (From the Source - This is NOT A required field) to a target field TargetSalary(Target field - This is a Mandatory field).

When the source field Salary elment tag is not there the Mapping fails, because the target field is a Mandatory field.

To over come this I used the Exists function with Source Salary field, and if it returns false, then I am sending a constant 0 to the target field, but this never worked. could some one guide me please.

Thanks