Welcome to the forum.
After some consideration I have formed the view that you have had an interview for a postdoctoral position that was inter-personally very agreeable.
So agreeable in fact that both parties left the discussion feeling quite confident that they could work together. We can all relate to this: being at ease with people in an intense working environment is almost a human necessity and a free-flowing line of communication a tremendous boon. This being rather hard to find, it is not surprising that when we do find it we may feel that everything else about that position, i.e. the actual work, the labs, library, canteen, colleagues, other faculty, office staff, sports facilities, the local city culture, etc will be at worst 'workable'. And this state of emotion may well make us neglect to run through that "matrix of job considerations" we may have mentally prepared on the train to the interview.
Yet, just as for every moonlight kiss there is a sunrise of reality, there comes a time in every fellowship when a client's needs must be ascertained or inferred, a real work schedule lain out and a set of project milestones set in place towards the eventual completion of the task.
Were you someone who had a difficult PhD supervisor, you no doubt would be quite happy to have found a research group leader whom you could work smoothly with and perhaps quite happy to let him/her determine the work objectives. Yet this does not appear to be the case with you: you had a seemingly reasonable supervisor and have a quite fixed idea as to what area you want to do research in. Making matters worse, the latter does not appear to be what this research group has in mind for your fellowship term.
Obviously, you will be fearing that your intense interest in your own favorite area will turn to intense frustration were you to continue on your present path. And this is surely the last thing that any researcher wants: research work always has an elegant sufficiency of frustration without one adding to it.
It may be helpful therefore to contact and respectfully apologize to the prospective research group leader for what seems to have been a rather unfortunate, although not uncommon, misunderstanding on the fellowship project objectives. Of advantage in this discussion is your own willingness to:
assure the PI of your professional regard for him and his research group
admit that only working in your preferred area is of real interest
acknowledge that hiring a Fellow with limited motivation for the project would be a disservice both to the work involved and the supporting university's commitment
In this way the parting would be respectable.
I could end this answer to you by wishing you well in your search for a fellowship that would be interesting to you.
But in writing that I would feel like I am being indulgent to you and ignoring what with decades of personal hindsight I recognize in your thinking here: I see an echo of some of my own foolish career decisions and fear for your prospects if you proceed as you seem to intend. It's kind of like that final parole application in The Shawshank Redemption where Morgan Freeman's character says he wishes he could talk to that 16 year old boy that he was before he went out to rob a liquor store and kill the attendant in the process, putting himself in jail for life as a result.
If you are fortunate enough to have come through the challenges - financial and intellectual - of a PhD programme and have an offer of a fellowship in a good respectable research group led by someone you get along well with (maybe too well!) then maybe it's time you manned up and humbly accepted that offer.
Life's not just about doing interesting things and hoping the euphoria of that will carry us through the rest of it. It's mostly about persevering through humdrum work, sifting out the odd nugget of use to the world and carrying on. Most folks don't get a fraction of the opportunities you can have with this job. Jeff Greenfield recounts here the happenstance preceding some of the major events of his life: they all seem to follow a "nothing" invitation from an agreeable person. (Of course, Greenfield is being modest about his own impressive analytical and communication skills as a young man: you don't get invites to high-powered dinner parties or to join RFK's campaign without real class)
Your offer of a fellowship "next year" may be genuine - or it may be a non-confrontational way of steering your out of your previous group. If you do find a position in the area you like, it may not be under a PI that you like. So please think hard about taking the fellowship offered to you and providing your own motivation to get through the less interesting aspects of it.