We wouldn't and according to https://www.nbcsports.com/nfl/profootballtalk/rumor-mill/news/darius-slay-wont-commit-as-to-whether-he-would-have-reported-to-the-eagles we'd get money.
The other wrinkle relates to the ability of the Bills to play hardball with Slay. If Slay fails to report, a portion of his $8.745 million signing bonus could be recovered by the Bills, even though the Bills paid none of it. It’s a simple proposition. The signing bonus represents advance compensation for future services. Failing to provide those services exposes the player to losing some of that compensation.