Payment Term Discount and taxes

Hi To all
we want to implement payment terms discount and we almost have it working, all seems fine, but taxes.

let me explain

we have a payment term like this 30 Days Net Due and 18 Days for a 3% discount

we make a invoice for 1500 € and the system is properly showing 45 €  of available discount

this is a intracommunitary operation (inside Euro Zone).

we have the tax area rate defined like this

authority      %
999999        0
999997       -18
999997       +18


999999 is the spanish standard taxes
999997 is the spanish intracomunitary taxes

when we post the invoice, the system is adding a row to F0018 with the right information for all 3 tax authorities