You seem to be under the impression that at and before 396 the city had a standing, volunteer army. This impression is mistaken. The Republican army, going back as far as the kingship, was a levied, seasonal force. Like the Greeks to the south and the Etruscans to the north the Romans of the early city called out citizen levies for the defense of the city when necessary, conscripting those citizens who met the necessary property requirements. Already by the Punic Wars the property requirement had become obsolete, as the minimum continued to fall in value and individuals falling below the nominal minimum could and did still volunteer for military duties. Marius formally did away with the property requirement entirely, but the army was still not--despite what you might read on the internet--a volunteer force. Armies continued to be composed of volunteers and levied troops throughout the late Republic, and the state never formally renounced the duty of the Roman citizen to defend the city--Augustus, famously, conscripted several outfits following Varus' failure. Prior to Veii there had never been a need to pay the army. It was composed of conscripted landowners who marched out for a season in defense of the city and then returned to their fields for the harvest. Even at Athens, with its rich supply of surplus tribute, the question of soldiers' pay was only raised quite late, necessitated by the increasing size of the standing fleet and the need to maintain military outposts throughout the empire for years at a time