You have to activate T-cash purchase confirmations:
1. Open the game settings in the top left corner of the screen.
2. Go to Additional Settings.
3. Tick Use Cash in the Confirmation window section.

From now on a confirmation window will pop up every time you want to purchase something with T-cash.

ATTENTION! This option won't work after another confirmation window has already popped up (e.g., when you don't have enough materials to complete an action). You won't be asked for additional confirmation in those cases.