I had this exact thing happen to me on my 2016 genesis. They put 0 in the turn in fee that time. I wouldn’t sign another. After two weeks owner of dealership called me and said he would send a check for $400 to sign a new one so Hyundai would process the paperwork. Fact is I have their car and a signed lease. Not my issue. I am sensitive to the dealers pickle they are in and they gave me a good deal and good service. I told them to ask genesis to accept the contract as I signed what was put in front of me.The deal could probably be undone but @nellie1289 got a good deal so for $300 it probably is just worth signing a new lease especially since if it would have been there it likely would have not been a dealbreaker. I would ask the dealer for a check for $300, maybe they will do it since it was their fault the purchasing option fee wasn't listed.
Side note : it is outrageous they want to charge you $300 these days to purchase a car out of a lease. This is ridiculous nickel and diming.