The passage is in Wars II. xxviii. 13
"And Alamoundaras captured one of the sons of Arethas in a sudden raid while he was pasturing horses, and straightway sacrificed him to Aphrodite;"
I'm both surprised by such a casual references to a sacrifice to a pagan deity in this era (maybe I shouldn't be?) and surprised that Alamoundaras sacrificed to Aphrodite of all gods, though I wonder if Procopius was using a Greek name for what was really a foreign god.
Any clarification is greatly appreciated!
Aphrodite had by this point long been identified with the goddess al-'Uzza, an Arab goddess; we have inscriptions dating back to the 1st century that amalgamate the two. Although human sacrifice was forbidden in Greek (and Roman) religion, it was not in pre-Islamic Arab polytheism. As per Vassilios Christides on a separate issue (thank you Google Scholar!), Syriac sources point to Alamoundaras (or Mundhir) sacrificing to al-'Uzza at around the same time; given the existing identification of the two goddesses, this would be a reasonable conclusion to draw; Procopius is well known for Classicising!
545 AD is a century too early for Islam, and many other Eastern countries still practised polytheistic religions at this point.