Start any stage in Story mode, then create an "Interim Save" and choose "Quit". Select the "Continue" option, then exit the map (Triangle) to go back to the previous screen. Select "Free mode" to cancel your Interim Save, then select a different character then you are using in Story mode. Next, buy any items you want from the shop, then exit the shop and hold Start + Select at the stage selection screen to do a soft reset. You should now have all your gold back and still have the items you bought.
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