How is it conceivable that Ibn Abbas would exercise ijtihad concerning something for which there is an explicit ruling from the Messenger of Allah (peace and blessings be upon him) forbidding masturbation? And should his concession be adopted, or does it contradict the Messenger's saying?

From God Almighty's wisdom, people vary in their understandings and perceptions. He made the Sharia evidence open to more than one interpretation, which led to differences among scholars in the jurisprudential subsidiary issues, such as the issue of masturbation.

Nevertheless, no one's statement takes precedence over the statement of the Messenger of Allah, peace and blessings be upon him. So, if a Muslim believes that the Prophet did not permit a certain matter, then the permission of others does not change the ruling. This does not contradict the fact that the mujtahid (one who exercises independent reasoning) is rewarded whether he is correct or mistaken.