Additional payment to homeloan not being available

In December I logged into the fnb app. Under detailed balance, it stated that the next installment date for my homeloan would be in jan, not Dec. I was unsure if I would be debited or not in Dec so I made a payment to my homeloan, expecting that if I was still debited then I could simply move the money back to my cheque account as I have a flexi option. I was still debited in Dec however the additional money which I had paid was not shown in my available balance. I was thus overdrawn on my cheque account. I enquired with my personal banker and I was told I either needed an overdraft facility or could apply to reverse the debit order which would take about 10 days. I don't want an overdraft facility and if I only transfer the cash back in 10 days then I would be fined for an overdrawn account. Why can't I just withdraw the additional overpaid cash from my flexi loan? I had to transfer cash from my credit card which will cost me transfer costs and interest now.