What XML package are you using for that category? The entity.gridwithprices.xml.config XML package will show the sale price by default (if there is one).
If this is a custom XML package, you can use aspdnsf:GetVariantPrice to get both the normal and sale price:
Code:
<xsl:value-of select="aspdnsf:GetVariantPrice(VariantID, HidePriceUntilCart, Price, SalePrice, ExtendedPrice, Points, $pSalesPromptName, TaxClassID)" disable-output-escaping="yes"/>
...or aspdnsf:GetMLValue to get just the sale price:
Code:
<xsl:value-of select="aspdnsf:GetMLValue(SalePrice)" disable-output-escaping="yes" />