Does Not stick to Verbal Quote

Mid September had problems with Bantam engine. This is a 1300cc 2005 model Bantam bakkie. Suspected head gasket. Took car to Rod Mitchell at Prompt Auto. He told me I had to have a total engine rebuild which would cost R15 000. I asked Rod how long it would take and he told me about 10 days. I explained to him again that I only had R15 000 and gave him the go ahead to do the job. I went there after 10 days to see how the job was progressing, and it had not progressed at all. Again I stated that I only had R15 000 and asked Rod if he understood that he could not go over that amount. On 28 Oct 2015 he told me my car was ready. I informed him on Tue 10 Nov 2015 that I had paid him his R15 000 and I would be collecting my vehicle the following day. He asked me if I had received an invoice. I had not. He then emailed an invoice to me for R21 884 - 45% more than the verbal agreement. I went to discuss this with him but he was adamant about the price. I returned to him today to collect my car, but he refused to give me the keys and stated the bill needs to be paid before I can take the car.