ABSA not wanting to use my additional payments to reduce total capital owed to save on interest paid

ABSA Vehicle finance is refusing to use my extra payments to reduce capital owed so that I save on interest paid. The options that they are giving don't work for me as I do not save on interest which are paying towards my residual or reducing the term or reducing installment. Portia and her Manager Nolan Dekoker are also not helpful, Dekoker is telling me about the terms and conditions arrogantly. All I am explaining to him is that he must keep my installment as is, keep the residual as is and use the additional payment to reduce the total amount owed to save on interest.