This text comes from Quintus Curtius Rufus Historiae Alexandri Magni, book 3, chapter 5 (emphasis mine):
Mediam Cydnus amnis, de quo paulo ante dictum est, interfluit. Et tunc aestas erat, cuius calor non aliam magis quam Ciliciae oram vapore solis accendit, et diei fervidissimum tempus esse coeperat. Pulvere simul ac sudore perfusum regem invitavit liquor fluminis, ut calidum adhuc corpus ablueret; itaque, veste deposita, in conspectu agminis — decorum quoque futurum ratus, si ostendisset suis levi ac parabili cultu corporis se esse contentum — descendit in flumen.
I've read a translation to Catalan of the particular sentence where pulvere and sudore appear, so its meaning is not a problem for me. The translation in Catalan of the expression pulvere simul ac sudore perfusum is "cobert de pols i ensems de suor". In Spanish would be "cubierto a la vez de polvo y de sudor" and in Italian "coperto contemporaneamente di polvere e di sudore". Such translations to some Romance languages lead me to think that the genitive should be used for the words in boldface in the text. A similar construction in which genitive is used, that I found in the book Método de Latín II: Incorpora Clave y Vocabulario Latino by Santiago Segura Munguía, is
Auri amore scelus commissit.
I cannot understand why the author uses the ablative case for such words.