difference between two words for balance of account
余额 means credit balance, the amount of money remaining in your credit card account.

For 尾数, it usually means when you buy something which is not available yet, the shop may require you to pay some money first (deposit), when the goods available, you will need to pay the remaining amount of the price, that is 尾数.

So I think the 尾数 on your bill means the amount of money you not yet paid.