For vouchers, you need to loop the salesPayments and do a test to see if the salesPayment is of type voucher payment method (10).
{xsl:for-each select="in/salesOrder/salesPayments/salesPayment"}
{xsl:if test="paymentMethod = 10"}
{xsl:value-of select="amount" /}
{xsl:if}
{/xsl:for-each}
A coupon is not a voucher and it has no dollar amount attached to it.
Thanks,