Vincent's NT Word Studies
16. I consent (sumfhmi). Lit., speak together with; concur with, since the law also does not desire what I do. Only here in the New Testament.Good (kalov). See on John x. 11, 32; Matt. xxvi. 10; Jas. ii. 7. Morally excellent.
Robertson's NT Word Studies
7:16 {I consent unto the law} (sunfemi twi nomwi). Old verb, here only in N.T., with associative instrumental case. "I speak with." My wanting (qelw) to do the opposite of what I do proves my acceptance of God's law as good (kalos).