interest on a retainage fee
I am a small contractor who did excavation work for a large company. I had a retainage fee withheld for my work which was to be paid October 1 of 2019. I finally received payment on May 4, 2020. The company took off all my interest charges for the late payments. They told me because it is a retainage fee I can not charge the interest. Can I charge interest and are they required to pay it. The work took place in South Dakota.