So click on the bill or item and on the right hand menu you'll see create a credit note. If you haven't created anything then create a bill for the amount and save (that will allow you to reconcile (match) with the money that left your bank account). Now that money is back in your account. So click on that bill and on the right of the bill you'll see menu and you'll see credit note. Click that and follow the instructions. Creating the credit note is telling revenue and your accounting product that the money is back in. Credit notes mean the money isn't income therefore no tax liability. Make sense?