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).
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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.
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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.
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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.
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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?
These short forms of せる and させる are ‘preferred’ in the Kansai dialect, but are not a requirement. The standard forms may still occasionally be heard.