Just tell the hr you got better offer and reject it. I dont think it will affect your future since you havent work wif them before they wont waste their time to anti u. Im sure they haf other candidates in mind and u reject they will just take the next one.

When u start work u still under probation and they will still evaluate u. Got chance they dont want u at end of probation

It will be burden a bit to find replacement. But the good thing is u havent even started. So they already have a list of candidates from the interview. You're not the only person qualified for the job. Im sure they have many back ups.

Also remember that under probation they can choose to not confirm u. So both parties are there to evaluate each other. U find better offer and run is the same as they find u not good enough and not confirm u. If they r good employer they will understand

Now u can get higher salary job will u donate by buying nutella from me