Abner wanted to transfer his allegiance to David, the latter gave him the condition to
Answer Details
Abner was the commander of Saul's army and had fallen out with Ishbosheth, Saul's son and successor. Abner then decided to transfer his allegiance to David. David agreed to this on the condition that Abner would bring Michal, David's wife, back to him. Michal had been given to another man by Saul when David fled from him. So, David wanted his wife back as a sign of Abner's loyalty to him.