and the time at which it will be realised is likewise unknown—

This belongs to the class of uncertain sales whose harm and hidden pitfalls are many and which, in the end, lead to disputes.

Points derived from the ḥadīth:
1- The sale is forbidden according to both explanations: under the first because the term of payment is unknown, and under the second because the object of sale is absent and indeterminate.
2- The text specifies this pre-Islamic practice, yet the ruling is in fact general for every sale that involves ambiguity and excessive uncertainty.
3- The wisdom behind the prohibition is that it is a gharar-based transaction that leads to gambling and the wrongful consumption of wealth, bringing in its wake quarrels, disputes, enmity and hatred.

A concise principle on prohibited transactions
from the words of Shaykh al-Islām Ibn Taymiyyah

The fundamental rule is that Allah has forbidden in His Book the unlawful consumption of one another’s wealth. He censured the rabbis and monks who devour people’s wealth wrongfully and reproached the Jews for taking ribā (usury) after they had been forbidden it. This applies to everything seized unjustly in exchanges, donations, or by taking it without the owner’s consent.

With regard to exchanges, Allah mentions two ways of wrongfully consuming people’s wealth: ribā and maysir (gambling). Usury—which stands opposite to charity—is prohibited in Surahs al-Baqarah, Āl ʿImrān, al-Rūm, al-Mudaththir and al-Nisāʾ, while the ban on gambling is stated in Surah al-Māʾidah.

The Messenger of Allah ﷺ then elaborated on what Allah had stated in general terms: he prohibited sales involving gharar—transactions whose outcome is unknown—because they amount to gambling. Examples include selling a runaway slave or a horse or camel that has bolted.

As for ribā, its proscription in the Qur’an is sterner still, and the Prophet ﷺ listed it among the major sins, for only the needy resort to it: he takes one thousand in cash on condition that he repays one thousand two hundred later, something a well-off person would never do. The added amount is therefore an injustice inflicted on the one in need.