Trying to retain control of the domain would be difficult, also if the domain was a .uk, then you'd have the transfer fee to pay at the end of the year if the customer was to keep the domain.
If you registered the domain yourself, then the customer may not be happy at seeing your name rather than his.
I'd retain the customer by good service rather than freebies. By all means offer a freebie, but make it a freebie not a tie in. If you want a tie in, then offer a discount (even a small one) for yearly payment.
Getting customers is difficult, however a customer worth having would probably not be interested in a tied in freebie