As I know, you cannot change UOM on POS screen. This is not same as sales order. You can use Discount field on POS for this case. Also, you can use Pricing tab on Retail Item Card. You will use the same UOM but you can set different price per Minimum Quantity as Sales Price in standard NAV.
Maybe someone made customization about it, but standard LS Retail cannot use different UOM on POS.