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الجواب وباللہ التوفیق

Purchasing a vehicle through installments is not permissible as it involves an interest-based transaction. 

However, the scheme explained in your question to avoid income tax, will be permissible only if you desperately needed a car.

Secondly, you must have available with you the full amount of that car value.

Thirdly, that amount is certain to be taxed by the income tax authorities. Only then there is scope to buy in installments to avoid income tax.

In summary:  

1. if the car is not a compulsory requirement.
2. or its full amount is not available at hand.
3. or if its total value is below the Income Tax threshold.

Then it is not permissible to buy it in installments under the pretext of income tax application. Either a halal form of purchase should be adopted, or a pre-owned vehicle be purchased instead, with available financial resources.

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