Try something like this:
If {ELaborDist.det} = "E"
Then Sum({ELaborDist.amount}) Else
If {ELaborDist.det} = "D"
Then Sum({ELaborDist.amount}) * -1 And
If {ELaborDist.det} = "T" And {ELaborDist.det} = "D"
Then Sum({ELaborDist.amount}) Else
If {ELaborDist.det} = "T" And {ELaborDist.det} = "C"
Then Sum({ELaborDist.amount}) * -1
-Dell