Hi Daniel,
If i understand this correctly, you would want to take end of month snapshots of stock quantities. For this, the steps that I think would work are
1. Build a generic datasource on the table with the fields required
2. Load the data into first level DSO on BW. This would have a timestamp of the load date. The dataload would have to be a full load.
3. In case you want to do comparison analysis of stock quantities, you could compare it based on the timestamps.
You can also consider non cumulative key figures as in the case of inventory datasources and build your dataflow in that regards
Snehith