Let everything be clear. Khuda ka wasta hai PBO pe terms and condition parha karain.
It is clearly stated that price will be final at the time of delivery which has nothing to do with full payment or partial payment. Full payment is only accepted if they can deliver your car within 2 months otherwise partial payment. Full/Partial payment is done to avoid late delivery penalty and has nothing to do with price increase or get any affect.
Indus may decide to not charge you extra as a gesture of good faith for those people whose delivery is in current month and have already paid in full. It is different every time. They have charged tons of time new prices even with full delivery