Workers and firms care about the real wage (W/P), and not nominal wage. Workers do not care about how many dollars they receive but about how many goods they can buy with those dollars. Meanwhile, firms do not care about the nominal wage they pay but about the nominal wage (W) they pay relative to the price of the goods they sell (P). So, they also care about the real wage (W/P). Therefore, the expected price level affect the nominal wage.