Angry and confused

To Whom it may concern

I have required a settlement letter from Bruma Finance on the 19/6/2018 for loan reference number ********** 0 , I paid via EFT the same day 2833.70 which was the settlement amount , but I forgot to change the loan reference number ********** 0 to ********** 0 and immediately emailed them with proof of payment . They allocated the money to the correct loan reference number ********** 0 after a long fight and lots of emails . Today they phone me saying there’s already R96 interest coz I paid it in the wrong loan reference number ********** 0 what should I do pay it or what?