The game specifically warns you if you buy armour for a job you don't currently have active (i.e. Fending gear if you are a Black Mage right now) and won't warn you if you buy gear that your job can wear (Striking gear as a Monk). It is extremely unlikely that they will give you a refund option for mistakes like these in the future.

If you made a mistake, you made a mistake. You can temporarily use an i650 armour from Alliance Raids or upgrade i640 crafted armour into i650 until you have the tomes again. Consider playing tank a bit to make use of the armour you bought.