You splurged on a nice dinner out and paid the bill with a credit card but left a tip in cash. Then you checked your credit- card statement online the next day and found what looks like another tip added.

What's going on? Some credit cards and banks automatically add a 15 percent or 20 percent charge to a restaurant transaction when the charge is authorized — when you first hand over your card — to cover a potential tip.

The reason they're guessing at the amount is restaurants run through the card asking for authorization before you've added the tip.

That's what happened to one diner recently at Genki Sushi at Waikele Shopping Center when she paid $33.51 for dinner then found an additional $5 online. She called the restaurant and was told the bank holds the extra money to cover the potential tip.

The bank in this case was First Hawaiian Bank.

Ed Pei, executive vice president of the consumer banking group at First Hawaiian, said when the transaction is processed completely — usually within three to five days — the account reflects only the exact amount charged.

The phantom tip appears because the restaurant has two parts to the credit-card charge: first seeking authorization to complete the transaction; then the actual transaction being processed.

"They should be processing it for the actual amount of the charge," Pei said. "The restaurant has no right to process anything but what the cardholder agreed to."

He said the practice has been around for years and prompts an occasional complaint. Pei said it's up to the restaurant about whether to include the charge.

Genki Waikele manager Dustin Hara said customers do complain about finding the online charge — probably more than a hundred in the past six months at his restaurant alone. But he said most are relieved when the charge disappears.

Over at Genki Sushi's Hawai'i administrative office, Dana Kuboyama in Human Resources confirmed that a temporary hold is used to cover the potential tip.

Kuboyama said she learned from personal experience to fill out the receipt carefully. Several years ago, she and colleagues went out after work one night and split the bill for drinks. They paid the tip in cash, but she put the $150 tab on her debit card. When she checked her bill, she found another 20 percent tacked on and the $30 definitely caught her attention.

She urges customers to check their receipts and bills. If she's not leaving a tip on the card, she writes a 0 and runs a line through the tip area of the receipt and fills in a total to help reduce the chances for confusion or overcharges.

But not all banks are the same.

Bank of Hawaii spokesman Stafford Kiguchi said that since 2006, Bank of Hawaii has worked with its merchants to ensure that a tip increase was not automatically calculated in their credit-card terminals.

Kiguchi said the rules also apply to establishments such as taxicabs, hair salons and spas, which are all places where customers often add tips.

At the state Office of Consumer Protection, executive director Stephen Levins said he can understand why consumers resent the practice, although he had no record of complaints filed with his agency.

Levins said the hidden tips seem certain to alienate consumers. "At very least, it's going to cause grief for the merchants and confusion for their customers," he said.

"It just seems unfair to the consumer."

Reach Robbie Dingeman at rdingeman@honoluluadvertiser.com or 535-2429.