If you want extra currency you can always refund items from your inventory. The items you don't need anymore or gift items from your neighbors that you think useless to your Restaurant.
Stuffs refunded is on one-third basis of its original or standard price. If you want to sell back items to store just go to your storage and hold the item and drag it to cashier machine icon to the top left or after the icon of chairs and tables.
After dragging the item to cashier machine icon you will now go to screen like this (image above) you need to choose your final decision, Check button for accept, X button for cancel.