The "aspdnsf_AdjustInventory" stored proc is the one that handles the reduction of product's inventory when a purchase is made. From your Gateway.cs class, you can add the line below somewhere under AppLogic.ro_TXModeAuthOnly logic.
Code:
DB.ExecuteSQL("aspdnsf_AdjustInventory " + OrderNumber.ToString() + ",-1");
You also have to look into CaptureOrder.aspx.cs -> Gateway.OrderManagement_DoCapture() method, and make sure the software is not reducing inventory levels any more since you've done it during the AUTH purchases already. This can be a tricky modification, so be very careful of what you're doing as it could possible break the capture logic.