If you have agreed the new lower rental amount with the landlord but the listing page shows the original price, this isn't a problem. It's sensible to have confirmation of the new price in writing from the landlord (e.g. by email) to prevent disputes.
Once the holding deposit has been placed, the landlord has an option via the website to adjust the rental amount. They can also confirm the right price later at the contract-signing stage.
As long as you've both agreed in writing, everything should go ahead without any problems.