Adam Clarke on 2 Samuel 15:8

While I abode at Geshur in Syria - Geshur, the country of Talmai, was certainly not in Syria, but lay on the south of Canaan, in or near Edom, as is evident from Jdg 1:10; 1Sam 27:8; 2Sam 13:37. Hence it is probable that ארם Aram, Syria, is a mistake for אדם Edom; ד daleth and ר resh being easily interchangeable. Edom is the reading both of the Syriac and Arabic.

I will serve the Lord - Here he pretended to be a strict follower of Jehovah, even while he was in a heathen country; and now he desires liberty to go and perform a vow at Hebron, which he pretends to have made while he was resident at Geshur. And all this was the more perfectly to organize his system of rebellion against his venerable father.