Is it obligatory to offer expiation by feeding the poor for previous oaths for which the questioner fasted out of ignorance of the ruling, and for subsequent oaths, knowing that he does not receive a monthly salary?

What has become clear to you regarding the obligation of sequential order for the expiation of an oath is correct, due to the Almighty's saying: "Its expiation is the feeding of ten poor people from the average of what you feed your own families, or clothing them, or freeing a slave. But whoever finds none, then a fast of three days." (Al-Ma'idah: 89).

So, if your fasting occurred at a time when you possessed what was sufficient for the expiation, then your fasting is not valid, and you must feed or clothe. However, if you were unable to feed or clothe at that time, then your fasting is valid. And if other expiations become incumbent upon you, you must expiate them in the same sequential order.