Select a product on a product listing page (Catalog -> Products) in your store Admin area and open the product details page for editing.

Open the "Variants" tab for editing. Refer to the "Sale" column to add sale prices to product variants:

Set a sale price either in absolute value or in percentages.
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A sale price set for a product variant overrides all other possible discounts that may apply to the parent product.

Click "Save changes" to submit the new configuration.
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The product listing and product details pages in the storefront will display a discount set on the default product variant for products with product variants. Other discounts (if any) will display when viewing the available product variants on the product details page.
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