In the received reading tradition of the Ethiopian and Eritrean churches sappa is sounded exactly as saday is, so the classical value is not recoverable from how the script is read today, and it is reconstructed by comparison with the other Semitic languages rather than from any Ethiopian testimony.
What would resolve this A transcription of Ge'ez words into a contemporary script that did distinguish the sounds, or an Aksumite-period bilingual or transcribed text in which the letter's value can be read off directly.
OpenSemitic historical linguistics; Ethiopian epigraphyDivinity Atlas Open Questions Register, First Edition