He was with David. Eleazar. In 2Sa 23:11, 2 Samuel 23:12, Shammah also was with him. Note the plural verbs in next verse, "theyset. and [they] delivered it and [they] slew". This account is supplementary, not contradictory.
Pas-dammim. Probably Ephes-dammim, between Shocoh and Azekah (1 Samuel 17:1).
barley. In 2 Samuel 23:11, "lentiles". Therefore the field contained both: and the accounts are complementary, not contradictory.