change of debit order without concern

in 2016 i changed my ABSA home loan installment to 5000 a month and stated that this amount will only change if the interest rate increase in such a way that i need to pay more the 5000. to my surprise end of February 2018, 3700 was debited on my account without my concern. in April i then went to a absa branch in middelburg mpumalanga i called the absa call centre and laid a complain. no one bothered to call me back i also requested a recording where i gave someone from absa the permission to change the money that needs to be debited.