"In hand" is the correct answer, but I think "on hand" is also okay here.
When you have money in hand, it means you have the money with you (whether it is actually in your hand or not), and you are ready to buy anything at the moment right away.
When you have money on hand, it means you have the money, but it may still be in your bank account or at home, etc. You could spent it, it is somewhere where the money is available for you to get in little to no time at all.