I'm planning to break the cup, but I'm going to hide it from the shop owner.
That doesn't sound like a good idea. Is it an expensive cup?
Yes, it will cost me almost all the money I have
You shouldn't hide things that you broke. You should be honest with the shop owner about the item. If you lie and get caught you might get into more trouble and have to pay more for the item.
But I really need the money.
It's unfortunate that you broke the cup, but don't you think that you should confess that you did. The shop owner might be pleased that you told him and might not even make you pay. Either way, you'll feel better for doing the right thing.