Do I (or the person who I've referred to bittr) need to make a €250 transfer at once?

Nope! You have (or the person you've referred to bittr has) 90 days to convert at least €250 and it can be an aggregate of multiple transactions spread over 90 days. As soon as the €250 limit is reached, we'll confirm your satsback in Satsback.com so that it'll be available for payout!