FNB TRANSACTION CREATING OVERDRAFTS

I made a purchase in the first week on April, the money on the app stated that it was pending but was not accessible to me. meaning it was no longer available until such time that it is deducted. this past weekend my account is now in the negative R400 imagine. I don't know why this bank are so inconsiderate. Now it is as if i took an overdraft. Fnb keeps on disappointing