> You are far, far better off changing Xalan to Saxon...

Yes, but that's not always an option (e.g. vendor support only xalan
at the moment)

Does someone have an idea how to get this working in XPath 1 without For-loop?

<xsl:variable name="totalNumber" select="sum(for $a in $inputSet
return number(replace($a,',', '.')))"/>

Thanks