Some Rules Concerning Qurbani
If a person does not fulfil their Qurbani obligation in time, then giving its value as donation in money will not be a substitute.
If rich people do not fulfil their Qurbani obligation and the time of sacrifice passes, then they will need to give the value of the Qurbani in money as donation.
If a poor person who someone who has promised a Qurbani (Nazira) buys the animal and does not sacrifice it within the necessary time, then it is Wajib upon them to give the animal away as Sadaqa.
If after Sadaqa Fitra or Qurbani becomes Wajib upon a person, and that person loses wealth (i.e. becomes poor) then this obligation will not be withdrawn and they will have to fulfil it during their lifetime. It is Wajib upon them to either give the animal or its value as donation.
There are different views concerning buying an animal from the wealth of a rich child. The advised and accepted view is for the Qurbani to be sacrificed.