The machine already has a VOID mode for this. Turn the key to VOID position. Now every item entry will be negative, so just make the sale again and it will be reversed. then turn key back to REG mode.
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If anyone is looking for a Sharp XE-A106 cash register manual, c-tech collective have it here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp_XE-A106_operation_manual.html
Registerman
November 2010