Though Brian Vera is already set to be his opponent on August 3, Julio Cesar Chavez Jr has challenged Martin Murray to come to Mexico City for a fight on that date, hoping in turn to be able to make a fight for the interim WBC middleweight title.
Chavez (46-1-1, 32 KO) will always have a perceived massive advantage in Mexico, and there's also the issue of drug testing, which has been a problem for Chavez twice now in Nevada, once for a diuretic and once for marijuana. Texas and Mexico are two places Chavez likes to fight, neither of which exactly go hard on testing. Given Chavez's history, it would be understandable if future opponents are a bit wary of fighting him without having drug test assurances in advance, and it may be hard for them to trust Mexico, the WBC, or Texas, where they "forgot" to do testing after Chavez's fight with Marco Antonio Rubio in 2012.
That said, Murray (25-1-1, 11 KO) is in a sad sort of position right now. He won a lot of fans and support for his performance against Sergio Martinez in Argentina, and there were plenty of folks who thought he deserved the nod on the cards in that fight. In a more favorable world for him, he'd have some big money waiting back in the UK, but Hatton Promotions has no TV outlet over there, and Martin might have to make a go of it internationally if he can't leave Hatton and sign with Matchroom or Frank Warren. It's not Hatton's fault, really, that Sky chose to get in bed fully with Matchroom, but unless Ricky can strike a deal with Warren and BoxNation, he's out in the cold on domestic TV, which really harms his better fighters like Murray and Scott Quigg, notably.
I think it also goes without saying that most people would rather see Murray than Vera against Chavez, with due respect to the gritty and likable Vera.