The parens around the subtraction value aren't relevant, it's that you needed to do the subtraction outside the arguments for Sum().
In other words your final code
Sum ([SoftScape Total],[HardScape Total],[Aggregate Total],[Removal Total])-([Discount])
works, and this would also work
Sum ([SoftScape Total],[HardScape Total],[Aggregate Total],[Removal Total])-[Discount]
I don't know offhand if
Sum ([SoftScape Total],[HardScape Total],[Aggregate Total],[Removal Total],-[Discount])
would work, but I suspect not
Addendum - if for some reason you HAD to do it that way (no idea why you might, just go with it), you'd either put negative values in [Discount] or else create a field (called [Discount to Apply] or something) with a formula [Discount]*(-1) and use that in your sum formula:
Sum ([SoftScape Total],[HardScape Total],[Aggregate Total],[Removal Total],[Discount to Apply])
