It's worth mentioning that I'm using Filemaker Pro 11 Advanced. Thanks!
Much depends on your relationships between tables.
If [Inventory::Qty > 0 ]
Set Field [Inventory::Qty ; Inventory::Qty - 1]
Show Custom Dialog ["Item is out of stock"]
Might serve as your button, but I'm assuming a valid relationship from your current record to your Inventory table.
A conditional value list could be set up that eliminates items not currently on hand. See Option 1 in this link:
Thank you. My knowledge is way more basic that that wonderful equation you gave me. I'll disect the parts and figure out it this week. fingers crossed.
Well it's a script created in Manage Scripts, not an equation. It also requires modifying it to match your actual table and field names.