Do you have a condo fee? Write a letter with the Assoc name on the top, have you and the neighbor sign it and get it notarized. A 6D is a letter stating that all condo fees are current. Ask the closing Atty if he/she wants specific verbiage and you should be fine.
Any of the other owners who are trustees can sign the 6D. Your real estate attorney should have a blank form. Make sure you provide the closing attorney an original with an original signature. Good luck! (And stay close to your attorney on this process for more guidance.)
You may want to protect yourself and consult with an attorney who specializes in real estate for all related paperwork--or does the complex have an accountant handling the related finances, if so consult with him/her as well.