I have heard Anthony Seibold during an interview (Grange TV on YouTube) talk about the step up from being an assistant coach to a head coach, and he mentioned managing the salary cap as one of the extra requirements of being a head coach. It makes sense that the coach would have to have decent control over the salary cap so they can create the roster they want. I get that the board signs off on the contracts, but someone has to come up with a figure as to what a player is worth and I don't think board members are going to understand what a player is worth like a coach would.
Maybe the story is different for Wayne's recent stint at the Broncos, but the only thing I've read was an article in which Boyd and Bennett's agent (George Mimis) said that Bennett didn't have much to do with the details of Boyd's contract, other than wanting him around. I don't think George Mimis is going to throw Wayne under the bus and say yep, Wayne was the one who signed the out of form Boyd for 4 years, so he throws Paul White under the bus instead. I found an article from back when we signed him which reported that, "Wayne Bennett last night confirmed they have the funds to make Boyd a Bronco for life." I really don't think we can be certain how it all played out unless we were there, who ever really knows the details of what goes on behind closed boardroom doors?