DAX Help - complex calculation

Hello,

I am encountering a roadblock in the DAX code I am trying to develop and would appreciate any guidance or support! 

Here is a picture of the work in progress (see below). The first level of the matrix is the promotion description. The next level drills down to the individual items in that promotion. The goal of the report is to compare items sold on promotion against that same item's total sales. There are two measures that we need for the matrix:

Promotion Units: shows how many items sold on promotion

Total Units Sold: needs to show total unit sales for an item. As you can see we have a test measure that shows the total units sold which ignores whether or not a unit sold on promo. As long as >1 of the item sold on promo, we get the total unit sales. The problem is, I am unable to get this to aggregate at the promotion description level. The current measure titled total units sold is incorrect. The correct value should be 1391 for fresh fruit banana and as a total at the description level.

The data model is a star schema with transactions being the main fact table. There are three other dimensions being used: date, promotion, and item. Here is a picture of the model:

An item is on promotion if it has a promotion key in the transaction table. If it is null than the item is not included in the promotion. In this example only the top row would be counted as an item sold on promotion.

I need a way for the measure total units sold to aggregate the total units of any item that falls under a promo description. Is there a way to do this with DAX?

Hopefully that description makes sense. Thanks in advance for any help!