we paid in $-equiv: the Internet Archive calculated the price of bitcoins on April 1 and use that (rounded down to $100/btc). seemed fair.

I don't think they are paying the complete salary in BTCs. A part of their salary is paid in BTCs ( from the donations they received ).

That value definitely would be calculated based on the present value.

My guess, is that they just split the donations.

Nobody would be brave enough to pay (or work for) fixed bitcoin amounts right now.