The
shekel was originally a
weight, but once coinage developed the
tetradrachm was accepted, hence the '
shekel of Tyre', which is in fact a tet. Athenian tets were 'pure' silver (ie as pure as practical given the available technology), so they would have been acceptable; the restriction to Tyrian silver for the
Temple tax came about because of the debasement of the other tets. The same rule would have
applied here.
The Pidyon ha-Ben is explained here:
http://www.aish.com/jl/l/48964996.html ; the Torah passage relating to the five shekels is Numbers 18:15-16: The first issue of the womb of all creatures, human and animal, which is offered to the LORD, shall be yours; but the firstborn of human beings you shall redeem, and the firstborn of unclean
animals you shall redeem. Their redemption
price, reckoned from one month of age, you shall fix at five shekels of silver, according to the
shekel of the sanctuary (that is, twenty gerahs).
The five tetradrachms would have to be pure silver, otherwise you'd be shortchanging the Temple, which would have been extremely sinful! Athenian tets would no doubt have been acceptable, and as the common trade coinage -
Samaria, which was of course an Israelite kingdom, profited from trade routes from the Mediterranean to points east - they'd have been a perfectly normal offering.