All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The person liable for tax isn`t permitted to ask for paying it in installments with usurious interest since Allah prohibited usury and cursed the one who accepts it and whoever is involved in that. In this regard, Allah the Almighty Says (What means): "Those who devour usury will not stand except as stand one whom the Evil one by his touch Hath driven to madness. That is because they say: "Trade is like usury," but God hath permitted trade and forbidden usury. Those who after receiving direction from their Lord, desist, shall be pardoned for the past; their case is for God (to judge); but those who repeat (The offence) are companions of the Fire: They will abide therein (for ever)." {Al-Baqrah, 275}.
Moreover, narrated Jabir (PBUH):
Allah's Messenger (PBUH) cursed the one who accepts usury, the one who gives it, the one who records it and the two witnesses to it, saying, "They are all the same." [Reported by Muslim].
In conclusion, your (Asker) paying taxes in installments with usurious interest is religiously forbidden and having financial commitments doesn`t justify falling into sin. And Allah the Almighty knows best.