Inventory measures are usually modeled based on:

1. A periodic snapshot fact table at a daily or monthly date grain and sometimes.....

2. An accumulating snapshot done for the current month.

A semi-additive measure such as LastNonEmpty or LastChild is usually used to measure the inventory on hand at the end of each period.

One measure group would use the purchase orders fact table and....

One measure group would use the inventory fact table.

Are you SURE you do not have any historical data regarding inventory (no auditing history of previous quantities on hand?)