Q: I know a Muslim [female] who has been earning since a few years but recently came to know that Qurbani is wajib on her because she is earning. How can she compensate for all those previous years for which she didn't offer sacrifice.
A: Praise be to Allah.
The scholars (may Allah have mercy on them) differed with regard to the ruling on udhiyah: is it obligatory or Sunnah mu'akkadah (a confirmed Sunnah)? The correct view is that it is Sunnah mu'akkadah for the one who can afford it, but there is no sin on the one who does not do it.
If a person does not offer udhiyah although he can afford to do so, he/she is missing out on a lot of rewards that results from offering the sacrifice, but he/she is not sinning, and he does not have to make up what he missed in years past, because udhiyah is Sunnah and the time for it has gone. This is the case if he/she did not make a vow to offer a sacrifice, in which case it is obligatory for him/her to fulfill his/her vow and offer udhiyah.
It says in al-Mawsoo'ah al-Fiqhiyyah (34/46): "If a person does not offer a sacrifice before the time has passed, if it was a voluntary sacrifice he should not offer it; rather he has missed the opportunity to offer a sacrifice this year. But if it was a vow, he has to offer a sacrifice and make up what he was obliged to do."
Based on that, if she wants to offer udhiyah, then from now onwards she has to prepare ahead of time, so that when the next Eid al-Adha comes, in sha Allah, she will have prepared her sacrifice beforehand, so she will not have missed the time for udhiyah, as happened in the past.
We ask Allah to help us and you to do that which He loves and which pleases Him, and to accept righteous deeds from us and from you.
And Allah knows best.
Source; Islam QA