When Supplier Bar sends/allocates N more Foo Items to the Retailer, Retailer should increase its Supply of Item Foo by N. You can also attach a "type" to the Supply that you add.
Again - don't build this all by yourself!
But if you insist, I've seen consignment modeled 2 different ways and both worked fine. One way is to model all the consigned inventory as a separate distribution center. For example, if you have 45 physical stores and 5 warehouses, you'd have 50 distribution centers. You can treat all consigned goods as being held in a separate distribution center. This works particularly well if your suppliers don't actually send you the product, and the product is in fact shipped to the consumer from the supplier.
The other way I've seen this done is to use a type attribute on your Supply. Some example types could be "good condition", "slightly damaged", "safety", or in your case, "purchased" vs. "consigned." Therefore you could have rows like the following in your Supply entity:
id sku quantity type
-- ---------- -------- ---------
1 foo-item1 10 consigned
2 foo-item2 5 purchased
3 fooz-item1 20 consigned
Note that the sku must uniquely identify an item and if you have multiple suppliers for that item, then you need to be able to navigate from the sku back to the supplier.
And you can also find a reasonable open source data model for invoices and oders.