Well, this is different from the application I want in plenty of levels, you can check the twbx file I've uploaded.
As time is on the columns and I want only to take the difference between the first and the last sales, I've put this in the discrete level, else I guess I'm gonna see that number plenty of times.!
Also I thought that a good way to address this issue was to use the total, with a twist:
where Inventory Drop is the formula of difference between first and last sale, but it doesn't work when you have null dates between data I think