Haroldo:
I know of no direct buses but it's easy to change in the Argentine city of Resistencia. Three companies leave daily from Asuncion: El Pulqui at 11:30, Crucero del Norte at 13:00 and Plus Ultra at 15:00, arriving Resistencia 18:30, 19:25 and 21:30 respectively. Fare 40 pesos argentinos.
From Resistencia a Flecha/PDA bus leaves at 21:55, arriving Salta next morning at 09:55 -- the "cama" fare is $254 pesos.