****** by this collection company

I paid this people R1355 towards a debt of R3703,but i still owe them R3309,74, What happened to R961,74? Are they telling me that only R393,26 out of R1355 went towards the original debt? What kind of day light rubbery is that? Even if there are interest, what sort of interests would that be, i find this very ********* and taking advantage of people