NikonUSA today announced a deal that runs through September 27. You can take any working camera, regardless of brand or type, and get a US$100 savings on the Nikon Z5. That's in addition to whatever your get for the trade itself. As usual, this type of deal only works through Authorized Nikon Dealers, and only to US customers.
If you've already ordered a Z5, don't worry, NikonUSA will let you take advantage of the deal retroactively, so talk to your dealer. If you want to arrange this deal through NikonUSA, NikonUSA will pay for the return shipping on your trade-in (see the details on the linked page, above).