The reward for the Monthly Chronos Quest "Get 50,000 Drachmas from the Bank" is 20,000 Drachmas, not 50,000.
To complete the Quest, acquire 50,000 Drachmas from the Bank via purchases. Upon Quest completion, you can claim the reward of 20,000 Drachmas.
The 50,000 Drachmas purchased to fulfill the quest requirements are added to your balance right after payments. Also, you do not need to buy 50,000 Drachmas in one go or have 50,000 Drachmas on your balance at the moment of Quest completion. The Quest progress will count the purchased Drachmas even if you spend them right away.