Is Zakat obligatory on an amount of (40) thousand Riyals that has been held for a full year, if more than (60) thousand Riyals was spent from it on a surgical operation, and does the sin of not paying Zakat fall due to the lack of money?

The original ruling is that Zakat (definition) must be paid immediately upon the completion of the nisab (definition) (threshold) and the passing of a hawl (lunar year) over it. This is based on Allah's saying: (And give the Zakat), and a command implies immediacy. However, there is no sin upon you for not paying it due to your need for the money, as scholars have permitted delaying it in such a situation, especially if the owner is poor and needs it, and their sufficiency would be disrupted by paying it. Nevertheless, the Zakat remains a debt upon you, and you must pay it when it becomes easy for you to do so.