Is Zakat obligatory on money saved for marriage if it reaches the nisab, and on gold purchased as a dowry but the marriage contract was not concluded? Is the one who delays paying Zakat on time sinful? What is the ruling on Zakat for debts owed to him by his relatives?

The Zakat year is calculated according to the Hijri calendar.

The nisab (minimum taxable amount) for paper currency is estimated based on the lower value of the two precious metals (gold or silver). The nisab for gold is approximately 85 grams of pure gold, and the nisab for silver is approximately 595 grams of pure silver.

If the money reaches the nisab and a full Hijri year has passed over it, Zakat becomes immediately due. There is no expiation for delaying it except repentance and prompt payment.

Allocating money for specific needs does not prevent Zakat from being obligatory on it.

Debts owed to you are subject to Zakat. You have the choice to pay their Zakat either when their year becomes due or when you receive them for past years.

The gold you bought for your fiancée is subject to Zakat by you if it reaches the nisab, because it is in your possession and intended for sadaq (dowry). This is what the jurists of the Shafi'i and Maliki schools have explicitly stated.