You can't buy out signing bonus money, only the base salary.
For example, Andrew Ladd, who has a horrible contract structure, has three years left;
$3MM signing bonus, $1MM base salary, so that's 2/3 of the $1MM base salary, plus the $3MM, for a total of $4.667MM, and for three years
If the Islanders buyout Ladd, they still pay him $11MM of the $12MM owed, except, the $3MM signing bonus for this season was already paid, so it's $8MM of the $9MM still owed.
This is why some teams refuse to give signing bonus money in the contracts. Now for fun, check out this contract
https://www.capfriendly.com/players/auston-matthews
No one is buying out an elite player in their prime, this is just to see the extreme of signing bonus money structure.