"to repay them for the computer they bought." what computer? And just cut the cord, you've clearly made your mind. The sooner the better.

@Tymoteusz Paul I explained at #4 which computer.

Oh ok, missed that. Eitherway still the same applies, cut the cord, move on. Can't undo the pain, at least limit it.

Don't worry about the pc, they'll find a use for it. Nothing is final until you've all signed a contract, so don't feel guilty, life happens you have to move forwards.

You seem to be unclear about which job you really want. Until you decide that there isn't really much we can tell you.

@DJClayworth, I would like to join the new employer which came up later (I edited the text to make it more clear).

Also, don't even mention the computer. Again, this is business. Sure, they might be mildly annoyed, but they can just save it for their next hire or otherwise find a use for it.

It's not clear to me what stage of employment you are at in #3-4 with Company A behind offer 1 and Company B behind offer 2. Sounds like emailed offer accept of offer 1 and no response yet sent to offer 2?

@mxyzplk it is like you said.

Just out of curiosity: how can +60% before taxes lead to +80% after taxes?

If I understand it correctly you don't have a signed contract with company B. Don't withdraw your application with company A until you have a signed contract with company B. When B knows that you don't have any other choice they might change the contract details. Or they might find another person for the job. Of course you shouldn't sign a contract for A either, since you don't want to join them.

@Haukinger, different countries, different taxation.

"Am I risking to put my recommenders in a bad light?", yes, but for an 80% net salary raise I'd still consider doing it.

Important update at the bottom of my question.

@Wolf Did the computer have some special accommodation that was specific to you, or is it just a generic computer that would be needed by anyone doing that job?

Besides the points raised in the answers, depending on the countries (and since you mention EUR - this is almost certain) you have a very flexible way to close your contract in the probation period. This is in the case you would have already signed the contract and wanted to get out quickly (not your case, but adding this for completeness)

If you're going to do it, do it now. The absolute worst is taking on the new job and then leaving after a few weeks after your employer has invested time and effort to train you. Taking a couple weeks of pay and then leaving before you can materially contribute anything is the disaster you really want to avoid.

"they already bought a pc for my office" - for your (home) office, or their office... for you? If the latter you should return their property to them. If the former it's not your concern how many eggs they're counting as chickens. - You cannot renege on a contract that you haven't signed.