Pls I want fnb to pay back my R200 and tell me y they are doing that from the customer how will I trust them with my account

On the 06 of June 200rand was deducted on fnb account and I became worried and contacted fnb asking them to check ne then o talked with of their consultand and he told that on the 1st of January I paid rain 624 and fnb reserved that money and paid on the 06th now my question is y they reserved it and y they paid it one 06 and took extra 200rand from my account because I did not ask them to reserve it and he said he will ask them to reverse it and he is not sure wether they will agree to do that