A client asked to refund all the amount as he did not like the efforts

Hi Concern, I am a business owner, recently I acquired a client, We are into digital marketing services, we have taken him on a three month contract basis, he paid us 39500 + GST 18%, we have done 20 days of work with him, we went on 3 personal meetings with him as well, we modified the work according to his concern and we did it, now he said he didn't like our efforts, so we worked 20 days, According to our company, we charge 20k per month, but we have given him the offer pay us 13150 per month, which comes to 39,500. Now I said him like okay, sir, we will refund the money by keeping one month money and we will refund the money to you, according to our company, we charge 20k per month plus GST, 46650, he paid us for with GST, I said we will refund the money by removing our one month effort, He asks like a full money refund, how it is possible, sir you tell me? The mistake what we have done is we haven't made the agreement sir He is telling like I will go legally and I will send th