Here is an example of one way of getting the Absolute Value of some amount in Xslt. This uses the Xslt function format-number(num) to change the numerical value to a string, ignoring any negating hyphens. Once done, XPath function number(string) is called to return to a numerical value.
<xsl:value-of select="number(format-number($Value, '###0.0###;#'))"/>
In the format string, '###0.0###;#', the semi-colon separates the format patterns for positive and negative numbers. The hash following the semicolon indicates a copy of the positive format, meaning both positive and negative numbers will be formatted to the same string. Use XPath to turn the string back into a number.
Users of Xslt 2.0 can just call the abs(num) function.