If it is a matter of choice between udhiyah (sacrificial animal) and paying off debt on behalf of a poor person, then paying off the debt is BETTER, especially if the debtor is a relative.
 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) - Majmoo‘ Fataawa wa Rasaa’il Ibn ‘Uthaymeen, 13/1496