Is buying the sacrificial animal better, or paying off an installment of a debt with its price, knowing that the questioner has debts and is keen to pay them off?

If the debt is due, it must be paid before the sacrifice (Udhiyah). However, if it is deferred and one expects to be able to pay it when it becomes due, then there is no harm in offering the sacrifice. The incapable or the debtor is not obliged to offer the sacrifice, and they should prioritize paying off the debt before it.