Praise be to Allah.
Firstly: working out the nisaab [minimum threshold at which zakaah becomes due] for paper money
The nisaab in the case of paper money is to be worked out on the lower of the nisaabs for gold and silver.
The nisaab for silver is 595 grams and the nisaab for gold is 85 grams.
This was mentioned in a statement issued by the Islamic Fiqh Council of the Muslim World League, and a statement issued by the Council of Senior Scholars in Saudi Arabia. It is the view favoured by the Permanent Committee, Shaykh Ibn Baaz (may Allah have mercy on him) and others.
So whoever owns the nisaab must give zakaah, regardless of whether he is saving it to furnish an apartment, get married, go for Hajj, or anything else.
Secondly: zakaah on money that was put in a riba-based bank is due on the original amount only
If he put the money in a riba-based bank, zakaah is due on the original amount only, and no zakaah is due on the interest, because it is evil, unlawful wealth, and Allah is good and only accepts that which is good. What he must do is withdraw the money from the riba-based bank, or at least put it in a current account.
With regard to the interest received, it should not be kept in the first place, and he must get rid of it.
Please see the answer to question no. 251794 .
Thirdly: failing to give zakaah does not mean that it is waived with the passage of time
If someone fails to give zakaah for several years, he must still give it, and zakaah is not waived with the passage of time.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) was asked: There is someone who did not give zakaah for four years – what must he do?
He replied: This person is sinning by delaying his zakaah, because what the individual must do is give zakaah as soon as it becomes due, and not delay it, because the basic principle is that obligatory actions must be done immediately. This person must repent to Allah (may He be glorified in exalted) from this sin, and he must hasten to give zakaah for all the past years. None of that zakaah is waived. And he must repent and hasten to give it so as not to increase his sin by any further delay.
End quote from Majmoo‘ Fataawa ash-Shaykh Ibn ‘Uthaymeen (18/295).
What your husband must do is try to find out how much he had each year, then if it reaches the nisaab, he must pay zakaah on it. If he is not sure about the amount, he should err on the side of caution. He can go to the bank and request copies of statements, and find out the balance for every year, after deducting any interest received, if there is any.
Zakaah must be given immediately, and it is not permissible to not give it all at once, unless there is a valid excuse such as not having cash available, in which case he may wait until it is available.
He must repent to Allah (may He be exalted) for not giving zakaah for this length of time.
And Allah knows best.