Is there a way to buy a theme and have it registered to a client yet under my account for tax expenses purposed for the client?
All of document will be generated theme purchased account details info wise.
A theme is only able to be associated with the user account used to purchase the theme.
I would suggest you create a new Envato account for your client and use this account to purchase the theme with your billing information. This way you can claim it as a business expense when invoicing your client, but you can give them the login information (so if they ever need theme support in the future they can submit a request with the account used to purchase the theme).