is he not entitled to accept as its legal tender
Only when a debt has already arisen such as an account with the butcher which is settled at the end of a month.
In a normal shopping purchase, no contract exists & no debt exists until the money is exchanged, therefore a shop can refuse any form of payment without giving reason.
Similarly if a price is on an item and the store claims it is incorrect, the price is not legally binding on either party until payment has been exchanged.