This is a very great and logical question I’ve been asked several times. It’s important for us to understand that the Qurbani is a separate ritual altogether. It’s not an act of charity. It’s an actual act of Ibadah as instructed by Allah (SWT). So, when it comes to this particular act, the act of actually sacrificing an animal or delegating the sacrifice to someone is what is important and obviously we see in this that the intent is Taqwa as Allah says,
“ لَن يَنالَ اللَّهَ لُحومُها وَلا دِماؤُها وَلٰكِن يَنالُهُ التَّقوىٰ مِنكُم“
(Quran – Surah Al-Hajj 22:37)
…that the flesh and the blood of this animal is not going to reach Allah, it’s the consciousness that this sacrifice is for Allah that’s going to reach Allah (SWT) from us.
We have to understand that it’s not an act of charity – it’s an instruction that we are fulfilling of Allah (SWT) and therefore what we are going to use for the purchase of this animal are the extra funds (the surplus funds) that we have that are going to be enough to purchase the animal of our intent. Normally the Ullama say, if a person has the Nisab and through that Nisab you are able to purchase an animal for sacrifice, then you are going to be obligated to do so. It’s from your surplus funds that you’re going to fund this instruction of Allah (SWT).