How is the zakat calculated for money that a mother saved as a dowry for her disabled son, on which zakat has not been paid for years, knowing that part of it was loaned out? And is zakat obligatory on money owed?

We ask Allah to forgive your mother and have mercy on her. Saved money is subject to Zakat (definition) if it reaches the nisab (definition) and a full year has passed, even if it is for marriage or building a house. The nisab for currency is equivalent to 85 grams of gold or 595 grams of silver. It was obligatory upon your mother to pay Zakat every year, and you must now promptly pay it for all past years, calculating that to the best of your ability and diligence. The amount of Zakat is a quarter of a tenth (2.5% per hundred). Scholars differed on the Zakat of a loan; the preponderant view with us is that its Zakat is obligatory every year if it is owed by a wealthy and responsive person. However, if it is owed by someone in hardship or a defaulter, its Zakat is only obligatory for one year upon its retrieval. It is more prudent to pay it for all past years.