Transaction costs are taken from the Mifid ii feed. In some cases, due to reporting requirements which allow for delay impact, this can result in a negative transaction cost. The delay impact is caused by the difference between the price immediately before an order to trade is placed and the actual price at the time the trade is executed. If there has been an increase in the asset price between the time of order and execution, this results in a positive return for the fund resulting in a negative transaction cost.
See further articles around charging and calculations in our related articles below.