The device was originally in your organisation and you have the licence. You unclaimed the MX64 hardware and moved that hardware to a new organisation. That device is now sitting in your customers organisation without a licence.
You could try transferring one of your licences to your customer and back again - but I wouldn't do that. A compromise if you want to continue down this path is to create an extra network in your organisation and put the MX their. Then grant your customer access to that network. That will solve the licence issue and let the customer try out the MX.
But this is hard because you aren't using the approach that Meraki is recommending for trials. What you can do is use the Meraki Try and Buy program for your customer. Then everything will be easy for you and your customer.
https://meraki.cisco.com/form/trial