What about using the By-Products feature? Enable support for byproducts in the Configuration --> Settings Menu in the Manufacturing App. The "Tool" is actually regular storable product that is one of the components listed on the BoM of each final product. It is also a by-product of the Manufacturing Order (there is a new By-products tab on each BoM to manage this). The tool can be serial number tracked if there is a unique number for each one, and you can do an Inventory adjustment to bring in all the tools with each of their numbers to make them available for use. The product "cost" would be $0 since the expense of purchasing the tool is probably already recorded from when the tool was billed and paid for. The tool when it is a component will participate in the reservation logic (if you only have 4 tools, the fifth MO will have to wait for the first MO from the others to "make" a tool as a by-product and have it return to stock to be available for another reservation).