Is a lawyer's inability to return money taken from his client, due to his dire financial circumstances and his use of the money to cover his family's essential expenses, despite his sincere intention to repay it, considered a violation of Sharia?

Provisions are predestined and guaranteed by Allah Almighty. Abundance in wealth is not an indication of Allah's pleasure, just as scarcity is not an indication of His displeasure. Returning the money to its owner does not contradict Islamic law. The amount you took from the man is not permissible for you to spend on yourself without his permission. If you do so, you are liable for its guarantee.