The 差 in 差事 is pronounced chāi (1st tone), not chà (4th tone) or chā (1st tone). This is a special reading: 差 means 'to dispatch' here, as in 出差 (to go on a business trip).
history
差事 originated in imperial Chinese bureaucracy, referring to official duties assigned by superiors. It retains a slightly old-fashioned or colloquial flavor in modern usage.
