If the Lions are going to acquire Anthony Hargrove, it is going to have to be via a trade, because the defensive end is reportedly going to sign his tender with the Saints on Monday. What that means is that Hargrove will no longer be a free agent, forcing the Lions to make a trade if they want him. Considering the fact that the Saints have publicly stated that they want him back, I'd say the chances of him being dealt are pretty slim. This seems like the opposite of the situation with Rob Sims, who signed his tender to speed up a trade because teams are no longer limited to the compensation designated by his tender.
There was some speculation lately that the Lions might make Hargrove an offer that would include a "poison pill," which, as Tom Kowalski explains, would make it impossible for the Saints to match a deal to retain the defensive end.
A poison pill is a loophole in the current Collective Bargaining Agreement that allows a team to pull a fast one when signing a restricted free agent. For example, the Lions could sign Hargrove to a contract that pays him $3 million per season, but his salary escalates to $20 million per year if he plays four games per season in the state of Louisiana. The Saints, of course, couldn't match that contract and the Lions would get Hargrove.
If Hargrove does indeed sign his tender, that completely eliminates the possibility of a poison pill (though one wasn't necessarily likely anyways) because the Lions will no longer be able to offer him a contract. It's certainly possible that the Lions could negotiate with the Saints in hopes of trading for Hargrove, but as I already said, it doesn't exactly seem very likely to me.
The Saints have already shared their intention of keeping Hargrove in New Orleans, and although a sweet offer from the Lions could certainly make them rethink their position on Hargrove, I doubt Detroit would give up more than a third-rounder for him. After all, if the Lions really wanted him that badly a deal could have been worked out by now where the Lions would only end up losing a third-round pick based on the terms of his tender. What's more, a deal could have included a poison pill if Martin Mayhew really wanted to make sure Hargrove ended up in Detroit, so to me it seems like this was more of a case of the Lions exploring their options than anything.