ubiquitin,
“concept of Kohanim” arent kohanim. I f I redeem my bechor with a big tzadik who I say fullfils the “concept of khanim” you would say (correctly) that I didnt do the mitzvah
The Torah attests that Shem (Malchitzedek) had a form of kehuna. So yes, you are correct that there was no Aharon or bnei Aharon alive yet, so Avraham Avinu did not have access to the kohanim that we are supposed to give maaser to, but I don’t get how you say they could not have fulfilled the mitzva, since they did have access to a kohen.