A clothing retailer sells T-shirts at a price of EUR 10 each. The T-shirts cost the retailer EUR 5 each. Customers have the right to return T-shirts within four weeks of purchase, provided that the T-shirts are unworn and undamaged. Based on the historical evidence, the retailer expects that 10 per cent of the T-shirts will be returned, of which half will be subsequently resold by the retailer. 

When and what revenue should be recognised?