You're right, it was M. Donald Grant who decided to deal Seaver rather than pay him. I'm not sure about the circumstances surrounding the Nolan Ryan trade, but it proved to be a hasty/foolish decision.
It is one of those deals that obviously looks really bad in hindsight. for years, the Mets had a black hole at 3rd base and they traded for a guy who had been an All Star in the AL for about 6 or 7 years in Jim Fregosi. At the time, it did not look so bad. Fregosi was an established star. Ryan was a bit of an enigma, great arm, buy struggled to throw strikes.
Fregosi hit about .230 in less than two season with the Mets and we all know what Ryan did. It was one in a series of terrible trades that would make the Mets a perennial doormat through most of the 70s and early 80s (with the excpetion of one flukey season where they came within a game of winning the series in 73).