A refund has to be inserted into the CSV file according to exact requirements for it to be processed.
For this to be possible, the refund needs to be clearly assigned to a sale saved with Taxdoo. As soon as the refund can be clearly assigned to a sale, it will be further processed and available in the transaction data the next day (provided the refund is taxable in the current month). Please also note this article regarding the time from which the uploaded transactions are visible.
In order to correctly assign the refund to the sale, the refundNumber must be completed and the identifier, transactionNumber, itemNumber as well as itemPosition (if set) each need to correspond exactly to the values from the associated sale for channel (and source, if set).