Is a divorce sworn to by a person to himself, and about which there is doubt whether it was uttered, considered valid? And what is the ruling on someone who hypothetically uttered it and agreed to a certain amount, then the car owner reneged on his promise after the agreement?

The obsessive whispers (waswas) have greatly affected you. What we advise you to do is to completely disregard these whispers. No matter how much Satan whispers to you that you have made your wife's divorce conditional, turn away from what he casts into your heart, seek refuge in Allah from his evil and his whispers, and firmly and decisively believe that your wife is still under your marital bond. And whenever you doubt having uttered a conditional divorce, the default is the absence of what you doubted, so do not give this doubt any attention. Even if a person afflicted with waswas explicitly utters the word of divorce under the influence of waswas, this utterance is not considered valid, and divorce does not occur by it.