The Catholic idea that the priest has the authority to either forgive or retain is NOT found in the Greek meaning of the text. The word they have translated into "will be" or "are" is actually the Greek word that means "I am". As in it already existed. The priests do not have, not does anyone else, the authority to decide or commit heaven to their declaration. They, as well as we, have the authority to only declare what has already been stated in scripture.
I don't know enough Greek to evaluate the Greek words at issue; could you provide a link to a discussion of them?
But I can evaluate the English and note that Catholics do not translate this as "will be" but rather as "are". Here is John 20:23 as found in the Catholic RSV (from the EWTN website):
If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained."
And by the way, how would you deal with Matthew 16:19, which, even in the King James Bible is stated in the future tense:
And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven.
The Greek word used there is "esomai" which is the future tense of eimi and is best translated as "shall be".
http://classic.net.bible.org/strong.php?id=1510