The القرآن explicitly provides the alternative of fasting for الحجاج who cannot afford the sacrifice: 'But whoever cannot find (a sacrificial animal) — then a fast of three days during الحج and seven when you return home. That is ten complete days' (2:196). This alternative is available only when the الحاج genuinely cannot afford the sacrifice — not as an optional choice for those who have the means.
الشيخ ابن باز explained the timing of the three days during الحج: the best time is to fast the 6th, 7th, and 8th of Dhul-Hijjah (after entering الإحرام for الحج but before the Day of عرفة). The الحاج should not fast on the Day of عرفة (9th), as النبي مستحب not fasting on that day for الحجاج at عرفة so they have strength for worship. If the الحاج 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 الشيخ ابن عثيمين accepted this position.
The seven days are fasted after the الحاج returns home. They do not need to be consecutive, though fasting them consecutively is preferable. If the الحاج delays the three days and was unable to fast them during الحج, the majority of scholars hold they should still fast them (even after returning home), making a total of ten days fasted after الحج. الفوزان confirmed that the fasting alternative is only for those genuinely unable to afford the sacrifice, and if the الحاج later acquires the means, they should offer the sacrifice even if they already fasted.