If the sacrificial animal becomes damaged after selecting it
Question

I bought a sacrificial animal, and at the time of slaughtering it jumped from on top of the roof. We caught up with it and slaughtered it before it died. Is that counted as a sacrifice?.
Praise be to Allah.Firstly: 
If a person has selected a sacrificial animal, then some damage befalls it that is not caused by any wrongdoing or negligence, then he slaughters it at the time of slaughtering, then it is valid and is counted as a sacrifice. 
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (13/373): 
If the sacrificial animal is sound and free of faults, then something happens to it that renders it unsuitable, it should be slaughtered and it still counts as a sacrifice. This was narrated from ‘Ata’, al-Hasan, al-Nakha’i, al-Zuhri, al-Thawri, Maalik, al-Shaafa’i and Ishaaq. End quote. 
The evidence for that is the report narrated by al-Bayhaqi from Ibn al-Zubayr (may Allah be pleased with him), according to which a one-eyed she-camel was brought to him as a sacrificial animal, and he said: If this happened to it after you bought it, then go ahead (and sacrifice it), but if it happened to it before you bought it, then offer another one instead of it.” Al-Nawawi said in al-Majmoo’ (8/328): its isnaad is saheeh. 
Shaykh Ibn ‘Uthaymeen said in his essay Ahkaam al-Udhiyah (Rulings on the Sacrifice) concerning the rulings on selection of the sacrificial animal: 
If the animal has a fault that renders it unsuitable for sacrifice, one of two scenarios must apply: 
1 – That was caused by the owner’s wrongdoing or negligence, in which case it must be replaced with another animal that is the same or more perfect, because if the damage was caused by him, he must replace it with another one to be slaughtered instead, and the damaged animal still belongs to him, according to the sound scholarly opinion; he may do whatever he wants with it, sell it or whatever. 
2 – The fault was caused without any wrongdoing or negligence on the owner’s part. In that case he should sacrifice it and it counts as a sacrifice, because it is a trust that is in his keeping but it got damaged through no action or negligence on his part, so there is no sin on him and he does not have to replace it. End quote. 
Secondly: 
How is a sacrificial animal selected? 
The selection of the sacrificial animal may be verbal, such as saying, “This is a sacrificial animal.” 
If they buy it with the intention of offering it as a sacrifice, Abu Haneefah and Maalik (may Allah have mercy on them) are of the view that the selection has been made thereby. Al-Shaafa’i and Ahmad are of the view that the selection is not made thereby. 
The scholars of the Standing Committee are of the view that the selection is made when one buys the animal with the intention of offering it as a sacrifice. 
It says in Fataawa al-Lajnah al-Daa’imah, 11/402: 
The sacrificial animal is selected when one buys it with the intention of offering it as a sacrifice or when one selects it. End quote. 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said in Majmoo’ al-Fataawa (26/304): 
If a person buys a sacrificial animal and it gets damaged before slaughter, it may be slaughtered according to one of the two scholarly opinions. End quote. 
Based on the above, if you bought this sheep with the intention of offering a sacrifice, then it gets damaged without any wrongdoing or negligence on your part, then it is counted as a sacrifice, in sha Allah. 
And Allah knows best.