By retiring from the NFL at the age of 29, former Colts quarterback Andrew Luck has walked away from millions in future potential earnings. He'll also have to give back millions he's already received, if the Colts choose to force the issue.
Based on his most recent contract, Luck owes the Colts a total of $12.8 million, if he indeed does not play again. That sum comes from $6.4 million in unearned signing bonus attributable to this year, and another $6.4 million attributable to 2020.
Grossly underpaid relative to other franchise quarterbacks at $24.6 million (it was a record deal when it was signed), Luck was under contract through 2021. At some point before then, he quite possibly would have set another high-water mark, like he did when he signed his most recent deal.
Of course, he could still change his mind and return to the NFL. The Colts, unless they release him from the reserved/retired list (and it would be ludicrous to do that), would still hold his rights.