The President has the authority to declassify anything. If it's true that he did this, and I have no reason to take the Washington Post all that seriously, then he has the authority to do so.
LMAO, The POTUS can do many things not granted him/her in Art. II; but providing classified information which offers aid or comfort to a foreign power can be an impeachable offense, i.e. a high crime or misdemeanor. Also, Art. I, Sec. 9, clause 8 (the Emolument Clause) can be ground for impeachment too.
Tits on a bull, aka Speaker Ryan, may be engaged in nonfeasance on this issue, since money from foreign powers is going into Trump's coffers, and has been since Jan 20th, 2017.