What I want, is that if the unique ID number matches one that is in the inventory table (there are some cases that it may not be),
That does not sound like a good thing.
I'm not sure why you need the additional table to show removed items when you already have this info in your ItemUse table, but the following script would do what you request:
IF [ Not IsEmpty ( FilterValues ( ItemUse::CheckBoxField ; "Yes" ) ) and Not Isempty ( Inventory::UniqueID ) //"Yes" was selected ]
set Variable [ $ID ; value: ItemUse::UniqueID ]
Set Variable [ $Qty ; value: Itemuse::Qty ]
Go to Layout [ "removedItems" (removedItems) ]
Set Field [RemovedItems::UniqueID ; $ID ]
Set Field [ RemovedItems::Qty ; $Qty ]
Go to Layout [original layout ]
This script could be performed by the OnObjectModify trigger on the check box field.