I think you are referring to the rule that does not allow a team to give a player an extension for 6 months after trading for them.
However, that is only for extensions. I believe Paul would be signing a Bird Right's contract, which is not an extension. He can only sign this once he is a free agent (so, he needs to actually opt out, and wait until the signing period starts), but I believe he could be traded in March, and then sign a full Bird Rights contract on July 1st.