In most shops in Delhi you have to pay the exact amount for thing you buy. Its a fact that in most of the shops, even if the shop keeper has changes to return but first they will insist you to pay exact amount in cash. Also in some cases they will not sell you if they have to give large change.
For example, if you want to buy half litre of mango juice (Price is 25 rupees) and you give 100 rupees to the shopkeeper, then the shopkeeper is most likely to refuse you. It's because he has to give 75 rupees of change to you.
It may sounds weird if you are a non indian or not from Delhi but its a real fact in Delhi. So make sure to carry changes/ smaller currency to avoid refusal from the shopkeepers in Delhi.