I know it costs money, but please upgrade to a newer version if at all possible. FileMaker 6's days of usability are truly numbered and you don't want to get caught in a situation where your computer has died and all replacement systems you can find do not run FileMaker 6....
Search this forum for "inventory Log".
You should find a number of threads that discuss setting up a log table where each addition and removal of an item from inventory is logged as a separate record with a summary field used to compute current inventory levels. Since you have FileMaker 6, these threads discuss a summary option that I doubt is avaialable in your version that restarts a running total with each new group. If that's the case, you can still use this method, but will only be able to view one specific item's inventory log records at a time. An alternative that will work in FileMaker 6, would be to use a related table with a Sum function to show the current inventory for all items in your database.