You need a way for the measure to apply a filter context to the sum. You don't need to explicitly supply a value though. You can simply pass in the DimPromoSkus table as the filter context.
Remember that the CALCULATE function can take tables as filter arguments. Here, we provide DimPromoSkus as the filter so that we don't sum any values that don't flow through the specified relationship. So, for example, since A3 C1 isn't in DimPromoSkus, the corresponding row in SalesFact doesn't get included in the sum.