FNB honouring ***** PURCHASE(not debit) way beyond acc balance

So on the 3rd of March I get a purchase notification on from my fnb cheque account. The amount going off far exceeds the available balance on my account.

Instead of the the purchase not going through due to insufficient funds, it goes through and leaves my balance in a negative.

I do not have an overdraft with FNB, so why are they allowing purchases to go through when funds are insufficient?

I would understand honouring a debit order. But a purchase when funds are insufficient will never make sense to me.

I could’ve avoided this ***** situation unscathed yet I’m in a situation which will affect my credit score negatively because I had requested funds to balance my credit card but now I can’t because the money requested has now brought the balance closer to 0.

So essentially someone spends money that I don’t even have and I must pay for it later. Utter nonsense.

This is shocking and truly beyond comprehension.