What is the ruling of Islamic law regarding dealing with a homosexual dentist, after having committed an immoral act with him, and then repenting and feeling remorse, knowing that the dentist completed his treatment without requesting financial compensation? And what should I do as much as possible?

You must repent to Allah Almighty by abandoning the sin, regretting it, and resolving not to return to it, while cutting off every cause that leads to evil and guarding against the steps of Satan.

As for the tooth: If the dentist installed it without charge for an unlawful purpose, and some of that purpose was achieved, then you are not obligated to pay him anything. However, you should donate the value of the tooth's installation for the benefit of Muslims or to the poor and needy. This would complete your repentance and expiate your sin, based on the statement of Sheikh al-Islam Ibn Taymiyyah: "Whoever takes compensation for an unlawful object or a benefit that has been consumed, such as the wages of a wine carrier, or the wages of a cross maker, or the wages of a prostitute, etc., should donate it and repent from that unlawful act. His donation of the compensation will be an expiation for what he did, for this compensation is not permissible to benefit from because it is unlawful compensation, and it should not be returned to its owner because he has already received the compensation, so it should be donated."