Equivalent to CountIF Excel Function

[Qty Offsets] = -- calculated column, not a measure
var __customer = T[Customer Number]
var __amount = T[Sales Amount]
var __invNo = T[Invoice Number]
var __qtyOffsetsForAmountNotZero =
	countrows(
		filter(
			T,
			T[Customer Number] = __customer,
			T[Sales Amount] = -__amount
		)
	) + 0 -- This is needed to force BLANK into 0
var __qtyOffsetsForAmountEquaToZero =
	countrows(
		filter(
			T,
			T[Customer Number] = __customer,
			T[Sales Amount] = 0,
			T[Invoice Number] <> __invNo  
		)
	) + 0
var __qtyOffsets =
	if(
		__amount <> 0,
		__qtyOffsetsForAmountNotZero,
		__qtyOffsetsForAmountEquaToZero
	)
return
	__qtyOffsets
	
-- Bear in mind that if the amount is 0
-- then we have to make sure that the
-- invoice number is not equal to the
-- current invoice number. This, of course,
-- is not required if the amount <> 0.

Best

Darek