If there is a law saying that you have to take cash then I would think that limits freedom of contract, not the other way around. It's not like absolute freedom

If there is a law saying that you have to take cash then I would think that limits freedom of contract, not the other way around. It's not like absolute freedom of contract exists anywhere in the world, at least that I know of, e.g. you can't enter into a contract allowing someone to kill you, sell yourself into slavery etc. Sweden definitely have a lot of other restrictions to freedom of contract such as limitations on what you can agree to in an apartment lease or employment contract.