chipping a car

[mhouli]

I have located a 2006 audi A4 1.9tdi 115 bhp which i would like to buy. The garage man is telling me as a selling point that the car has being chipped up to 140bhp by the previous owner who had to sell for financial reasons. Is there any mechanical problems this may cause as I heard it can stress the engine and also would it effect the audi warranty if anything should go wrong.