This looks simple to me. He wanted the year off to not get hurt. I'm assuming he was going to be paid a lot more. But the team in the WFL he was going to, folded after the 1974 season. He screwed himself.
Danny White played for Memphis in '74 and '75. I've never even heard of the WFL.
WFL overpaid many NFL players luring them away in hopes to get fans and viewership. They were not very well funded, and the league and teams were not run correctly.
I believe a few Dolphins players were quick to sign first.
Hence, since then when other leagues started up, they tried not to follow that model. The USFL in the 80's, tried to do better, but still overpaid.
Herschel Walker was owed around $5 million, or maybe $1 million, can't remember, from the New Jersey Generals from a personal services contract. But they let him out of the contract so he could sign with Dallas. Otherwise he had to fulfill that contract and sit for a year.
I think now other leagues learned their lessons and are trying again in a much better way.