Yes, but this does not happen automatically and the end date for the original product/office should have passed for the customer to be able to request the partial refund. The itself is done via their online account following the same steps as for the usual (full) retainer refund described here, except for the available option will show partial retainer return.
In case of any issues with the request customers can reach out to the Account Helpdesk following this journey.
NOTE: There cannot be any unpaid invoices on the customer’s account for them to be able to request the partial return of retainer.
Reference: Global Policies & Approvals