The 助動詞（じょどうし） (auxiliary verbs) せる and させる will be modified in 関西弁（かんさいべん）, creating a new form that interacts with the 未然形（みぜんけい） (indeterminate form) of verbs. In the case of う - Verbs, this is the form that interacts with ない, not the form that interacts with う. In standard Japanese, the せる and させる forms are referred to as ‘causative’ (someone/thing is made to perform an action). This meaning stays the same in the Kansai dialect.
The kansai form of せる will be す (with う - Verbs), while させる will become さす (with る - Verbs, する, and くる).

• 赤（あか）ちゃんにベビーフードを食（た）べさす。
  I make the baby eat baby food.
• 息子（むすこ）を一人（ひとり）で買（か）い物（もの）に行（い）かすと無駄（むだ）遣（づか）いをする。
  If you make my son go shopping alone, he spends money frivolously.
• 私（わたし）は疲（つか）れたらすぐ妻（つま）に運転（うんてん）さす。
  When I get tired, I make my wife drive.
• 迎（むか）えに来（こ）さすの？歩（ある）いた方（ほう）が早（はや）ない？
  You're going to make them pick us up? Isn't it faster to walk home?