The objective of the work is to propose a formal model for the service composition corresponding to the process synthesis in health care systems and verify the model based on the rewards that the end user negotiates with a web based health care system. The work focuses on the user specified cost and completion time of the customized composition representing various treatment processes and the sub-processes along with their corresponding services in health care systems. The model considers the "rewards" for each and every state of the medical services based on the treatment process requirements. A State Link Matrix (SLM) is proposed to decide the correct choice of composition of the clinical services and it is derived from the coarse grained services in the business process and augmented with the next immediate process to satisfy the cost and completion time requirements. The model is formally verified as Continuous Time Markov Chain (CTMC) model using PRISM tool to bring out the minimum and maximum rewards in the service composition. The time bound process composition and the cost bound service composition are analyzed with the help of CTMC and Discrete Time Markov Chain (DTMC) models to explore the probabilistic and stochastic nature of the process-service linkage. The reward based treatment process and medical service composition model is tuned as per the SLM so as to satisfy the patient's requirements.