Trying to make sense of gendoc conditionals

[partnerwerk]

In a gendoc template for invoices I want to add a line that only shows up when we have a discount.
When I use the following I do not succeed.

{ifexists InventoryDetails.discount_amount != 0.000000}
{InventoryDetails.discount_percent} %
{/ifexists}

I tried various forms of comparisons ( > 0, !=0, !=NULL but either the line is shown independently of having a discount in the invoice or not, or it is not shown at all.

Do you have a hint what I could do?