Officially it's not intended (anyhow) to release human members from your party, nethertheless there are some ways meanwhile.
But these possibilities all will require that your the party member inn storage is ok and is not affected by any bugs, otherwise success is not guaranteed.
Possibility I:
- Fill the first page of the inn list with pets.
- Change on the second/third page (where Finala has to be) and press the release button - it will work.
Acutally it's sufficent that in the slot 3 (example) of the first page is a pet if you want ot release a char that is in the 3rd slot of any following storage page.
Possibility II:
Release party members with help of mods like
this one. There are other/smaller too doing the same, they will all enable the release button of the inn storage even for human party members.
So this is there similar to possibility I, but you don't have to buy additional pets and to sort the order in the inn.
Keep in mind, if you leave a party member in the inn storage until the level difference is 20 or more levels, the party member from the inn will automatically be up-leveled close to your hero when you re-add to your party.