To enter material cost on a job without entering a purchase or a bill, you can use a Catalog Allocation. This is useful for items used on a job from your inventory or stock that did not require a vendor interaction.
To allocate materials go to your Catalog module:
Click where it says Allocate Materials at the top of the screen:
This will pull a window that prompts for the job, phase, and material cost you would like to add:
Once submitted, this will be part of the material cost of the job, noted as Via catalog:
This can also be done from inside Plan & Track by clicking "Allocate from Catalog" under the materials section of a phase.