Lösung:
<xsl:key name="item" match="Item" use="SupplierArticleNumber"/>
<xsl:template match="/">
<xsl:if test="not(SALESINVOICE/LineInformation/Item[SalesPrice != key('item', SupplierArticleNumber)/SalesPrice])">
<xsl:apply-templates/>
</xsl:if>
</xsl:template>
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>