I'd like to explain some more on why your inventory goofs up when deleting an item used on a transaction in QuickBooks, @PatrickM2.
In QuickBooks Desktop, you can only delete an item if it's not associated with a transaction. Eliminating an item that is used on an entry (invoiced or not) will mess up your inventory since it was applied to the transaction that you've created.
Know that the item used on the receipt will add up to your inventory since the entry has been voided.
To properly deal with this situation, I recommend consulting your accountant. This is to ensure the right course of action for your business and keep your records accurate. Especially if you're dealing with old transactions, wherein the books have been closed or reconciled.
Lastly, I'm also adding these handy links for your guide with QuickBooks Inventory in case you need them in the future:
Feel free to let me know if you have other questions. I'll be more than happy to answer them for you. Have a nice day!
