In the event purchases are returned to the dealer by the purchaser or consumer after the tax imposed by this chapter has been collected or charged to the account of the purchaser, the dealer shall be entitled to reimbursement of the amount of tax so collected or charged by him, in the manner prescribed by the Tax Commissioner. The amount of tax so reimbursed to the dealer shall not, however, include the tax paid upon any cash retained by the dealer after such return of merchandise. In case the tax has not been remitted by the dealer, the dealer may deduct the same in submitting his return. The dealer shall be issued an official credit memorandum by the Tax Commissioner equal to the net amount remitted by the dealer for such tax collected. Such memorandum shall be accepted at full face value from the dealer to whom it is issued when such dealer remits subsequent taxes under the provisions of this chapter. In case the dealer has retired from business and has filed a final return, a refund of tax may be made if the dealer can establish that the tax was not due.