Ruling on purchasing certain goods to win a prize attached to them

Question:
Some commercial markets place prizes on certain food products, such as milk and others. Is it permissible for a Muslim to buy in order to obtain the prize? What is the ruling on one who buys and happens to win a prize, not with the intention of buying to win, but rather the prize came by chance? Is it permissible or not?

Answer:
If the prize is based on gambling, where whoever happens to have such a number or such a number gets such a thing, this is not permissible; because this is from gambling, from maysir (gambling).
As for if the prize is known, they say: whoever buys from us four times, we give him such a thing, we give him... and more, we give him such a product, known, with no shubhah (doubt); this is permissible, this means encouraging purchase from them, or: whoever buys from us as a regular customer, we reduce the price of the product by one riyal, or two riyals in the product, this is permissible; because it is known, there is no gharar (excessive uncertainty) or maysir (gambling).
As for if they put numbers: whoever happens to have such a number is given a radio, and whoever happens to have such a number is given a car, and whoever happens to have such a number is given a refrigerator; this is from maysir (gambling), from riba (usury or interest), it is not permissible.