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