The rumor was that Foley was upset that Mero was paid more than him in WCW. Don't know how true. Also, that was a big reason for the hatred of HHH in 95-96. Foley bashed Mero big time in his book.
Foley wrote in his book he liked Mero as a person but he didn't understand why he was making loads more money than him in WCW first and then later in the WWF too because Foley didn't see star potential. Foley took crazy bumps, was over, good on the mic etc and Mero wasn't yet got more money. Then IIRC they offered to put Foley in a program leading into WM against Mero and Foley turned it down because he felt it wasn't the right feud for him, it wouldn't do him any favours. That's when he did the tag team with Terry Funk I think.