Debit order on a wrong day

I have this personal loan from FNB that I have had for a while now I have always been paying it always on the 25th of every month since I took it - actually, that had Venn the arrangement that we have agreed on until recently. Today, my company paid some money into my account which was not part of my income as I get paid on the 25th needless to say I saw a debit order on my account on the 15th of the month. What happened to our arrangement because those funds were meant for something else? I'm very disappointed in this bank because they can't just go debiting people's accounts with no prior arrangement regardless of how worried about they are about none payments in December month. Not my problem.... I'm really angry and feeling desrespected... please sort this out or make sure that I don't get debited twice in one month.<br> <br> Thanks.