set a precedent

to establish a pattern; to set a policy that must be followed in future cases. I'll do what you ask this time, but it doesn't set a precedent. We've already set a precedent in matters such as these.

Establish a usage, tradition, or standard to be followed in the future. For example, He set a precedent by having the chaplain lead the academic procession. The word precedent here signifies a previous instance or legal decision upon which future instances are based, a usage dating from the early 1400s. In British and American law it more specifically refers to a legal decision that may be used as a standard in subsequent cases.