This is a classic example of the typical Data Modelling challenge.You have 2 facts (Sales and Inventory) that share more than one common key. There are 3 approaches to this situation:
1. Do nothing and leave the Synthetic key (this "solution" was suggested by John Witherspoon, and a representative from QlikTech confirmed that this type of a synthetic key will work just fine. Many QlikView practitioners (myself included) prefer not to leave synthetic keys, for better scalability and for the sake of "clean programming".
2. Create a Link Table - identify your common keys, and create a single key, built as a concatenation of the individual keys. Your Facts will be linked by the combo key, and your common keys will only be stored in the Link Table. You can probably find more information about Link Tables if you search this forum.
3. Create a single Concatenated Fact. Concatenate both of your Fact tables into a single fact. All the common keys will get combined into a single field, and other fields will be added accordingly. This is the most scalable and the best performing model, however you lose the direct association between the two facts - in other words, it will be less trivial to show Sales Data for a Part Number associated with certain Inventory Balance, or to show Inventory of all the Items sold to a specific Customer (if that's needed). For those requirements, you need to create additional associations, but that's definitely too much to explain in a single forum message...
