The قرآن explicitly provides the alternative of fasting for حجاج who cannot afford the sacrifice: 'But whoever cannot find (a sacrificial animal) — then a fast of three days during حج and seven when you return home. That is ten complete days' (2:196). This alternative is available only when the حاجی genuinely cannot afford the sacrifice — not as an optional choice for those who have the means.
Sheikh Ibn Baz explained the timing of the three days during حج: the best time is to fast the 6th, 7th, and 8th of Dhul-Hijjah (after entering احرام for حج but before the Day of عرفات). The حاجی should not fast on the Day of عرفات (9th), as نبی کریم مستحب not fasting on that day for حجاج at عرفات so they have strength for worship. If the حاجی could not fast before the 9th, the scholars differ on whether fasting on the days of Tashriq (11th, 12th, 13th) is permitted. Aisha and Ibn Umar narrated that fasting the days of Tashriq was permitted only for those who could not find a sacrifice, and Sheikh Ibn Uthaymeen accepted this position.
The seven days are fasted after the حاجی returns home. They do not need to be consecutive, though fasting them consecutively is preferable. If the حاجی delays the three days and was unable to fast them during حج, the majority of scholars hold they should still fast them (even after returning home), making a total of ten days fasted after حج. Al-Fawzan confirmed that the fasting alternative is only for those genuinely unable to afford the sacrifice, and if the حاجی later acquires the means, they should offer the sacrifice even if they already fasted.