Let's say he would usually do it for $100. He told you he'd do it for 50%. You gave him $50, you got two songs. You offered more. 25% more. You gave him $25, you got a third song.
Seems to me like because you aren't paying him his usual rate, he is putting off your project in favor of those paying him in full. It's not very honest, especially if he told you he would do it for cheaper than his usual rate. You deserve the same speed and quality of service as anyone else handing him money for a product.
Some people don't see it that way, however.