I would stay put with that offer.
Bush (and I take it we are talking about Reggie Bush) is doing all he can as an every-down back but he 's not built for that type of pounding on a week-to-week basis. Plus he has already missed playing time because of a knee injury so the risk of him being injured once again is a strong possibility. And with your RB depth you can't afford any RB injuries.
Pettigrew has an injury but is playing through it but the entire Lions team is underacheiving as a whole and not producing compared to last year's team. You have a capable TE in Hernandez so Petty is relegated to your bench. DMC has had his struggles to date but it is more of a learning curve for him and the OL with the new zone blocking scheme installed during the offseason. Once the Raiders offense starts clicking DMC will get his yards. Graham is a top tier TE but you can only start one in your lineup and that leaves Hernandez on the bench (unless Graham's ankle puts him out of commission once again).
I like your counter offer but doubt he goes for it with Charles involved. You can try it but don't be surprised if he rejects it.