There must be some relationship between the 2 entities, for example an attribute that you can use to search for the purchase based on something like a productid. Then yoou can retrieve this information with a icroflow via a retrieve from database actiion with an xpath like [$attribute_ofpurchase = $inventory/attribute_ofInventory]
Once you have the purchase you can change this by means of other activities in a microflow.
Adding an association wouold be a easier proces to implement this feature.