Pro Football Talk first reported the news on Harrison, while Slay previously posted on Instagram his intention to stay away.
Slay, a 2017 first-team All-Pro selection and a two-time Pro Bowler, will earn a base salary of $12 million in 2019 and $10 million in 2020.
Given the mandatory nature of the minicamp, Harrison and Slay are now subject to fines for each day missed without an excused absence.
According to the collective bargaining agreement, the Lions could fine each player $14,775 for missing the first day, $29,550 for the second day and $44,325 for the third day. Harrison and Slay could lose up to $88,650 for missing all three days.