Peter Mbonabucha knows how it feels to come up short at the cash register and have someone step in to make up the difference. It’s happened to him.So when Sam Hammes, 33, of Cedar Rapids, came up $12 short on a day that he’d left his wallet at home and a work friend had already loaned him $100 to get groceries, Mbonabucha chipped in that $12.“One time I ran out of money and did not have enough to pay for my groceries, and somebody came out and they paid the rest of my (bill),” said Mbonabucha, now 47, of Cedar Rapids.