[Buddy B]

$500.00 minus 414.18 equals $85.82

Since his receipt shows that he paid $49.32 for gas,

$85.82 minus 49.32 equals $36.50 spent before this gas purchase.

Something on the other receipts [not gas] may help knowing his original Gift Card number?

[Rte66]

It was probably his last fill-up at the same place. Or maybe groceries or stuff inside Wal-Mart.