How to put received inventory into stock from a PO that a customer has cancelled?
When handling Inventory, you never Job Track it as custom ordered and specific to a customer. That describes Noninventory, not Inventory.
Inventory is bought generically, to have in stock in anticipation to sell, and in quantity to have on hand. or to order more when needed.
Example: I keep Widgets on hand, usually around 100. You want 1,000. I might be placing a Dropship order using the Noninventory item for the full 1,000. Or, I am sending 100 from Stock, ordering 1,000 more, and will fulfill the rest of your order when they arrive, but that is Not job tracking that order, because this is My Inventory.
And the purchase of inventory is the same as any purchase for inventory. Just because the customer canceled the order, doesn't change If you made a purchase.