Dental Treatment Arranged Unwittingly

I usually receive treatment at a particular dental clinic, from a very expert doctor (Doctor A). About a week ago, I had a toothache and there was no appointment available at my regular clinic, so I went to another clinic (Doctor B). Then, Doctor B said that an extraction of the tooth and implant were needed, which would cost me 5000 NIS, and he could do it in one session. As there are hardly any doctors in the city who do implants etc., I was convinced. The doctor asked me if he should take measurements, and I said yes. I agreed to pay in ten installments. Later, I decided that I am not willing to undergo such a complex treatment except with Doctor A — the expert I usually see. I spoke with him, and he indicated that due to the current condition of the tooth, it was not possible to do the treatment in one session as Doctor B had said. I called Doctor B to inform him that I did not want to be treated by him, and he said that I had to pay him one installment out of the ten, that is, 500 NIS. Do I have to pay him this amount?

Hello,

Since the doctor has already begun his work, you are obligated to pay this installment.

[In principle, he could have insisted that you complete the entire treatment with him, as this was agreed upon, and he had started his work].