I lent a relative fifty thousand, and I do not know when this amount will be returned. Is zakat due on it?

Regarding zakat on wealth, we say zakat is due on the wealth itself, not on individuals. Therefore, it is on the wealth, whether it is with you or with the person you lent it to.

But regarding who must pay zakat on that wealth, the zakat is on the owner of that wealth. We say if the borrower is wealthy, meaning he took that loan,

and that is for trade, investment, and similar purposes, but in reality, he is wealthy.

So whenever you want to reclaim that wealth, for example by taking it back, then you pay zakat for each year. This is like the case of a deposit.

This is like a person who lends someone money for investment in shares, for example in a shop or something similar, and the man has the money and can return it at any moment it is requested.

So we say at that point: and the entrusted deposit is given zakat every year.

But if that debt is owed by a poor, weak man who may not be able to repay that money.

Maybe you can get it back this year or the next, or something like that, so we say in that case, zakat is given once when it is received, even if it is only received after ten years.

The difference then is that zakat is given once if it is owed by a poor person, but if it is owed by a wealthy person and you can get it back at any time, then zakat is given every year. And Allah knows best.