If you have agreed the new lower rental amount with the tenant this isn't a problem. It's sensible at which ever stage in the application you are to have confirmation of the new price in writing with the tenant (e.g. by email) to prevent disputes.
Before the holding deposit has been placed
If this has been agreed before the holding deposit has been placed you can change the rental amount on your listing by heading to the 'Edit' page from your account. This will reflect on OpenRent within the hour, and your applicants are able to place the holding deposit on the property and continue as normal through Rent Now.
What if the holding deposit has already been placed? Or you don't want to advertise the lower rent?
Once the holding deposit has been placed, you have the option via the website to adjust the rental amount. You 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.