In December 1971 the Boston Red Sox traded 27 year old first baseman George Scott to the Milwaukee Brewers.
In December 1976 the Boston Red Sox traded part time DH and first baseman Cecil Cooper (who turned 27 two weeks later), to the Milwaukee Brewers for 32 year old George Scott.
Looked at solely from a first baseman to first baseman comparison the trades were a disaster for the Red Sox. From 1972 through 1983, Brewers first basemen accumulated 52.4 WAR (4.4 average/season) and averaged 136 OPS+ compared to 22.4 WAR (1.9 average/season) and a 106 OPS+ average. They managed to let the Brewers have Scott during his peak years, getting him back as he entered a speedy decline and trading Cooper just before he emerged as a star.
The deals look even worse when you just look at the years when the Sox had someone other than Carl Yastrzemski as their primary first baseman. Yaz played first from 1973 through 1976 and while Scott was a bit better with WAR of 17.5 and OPS+ of 133, Yaz was not too far behind with 14.9 WAR and 128 OPS+.
But look at 1972 and 1977-83. Over those eight seasons, Brewers first baseman accumulated 34.9 WAR and 137 OPS+ compared to 7.5 WAR and 95 OPS+ for the Sox. Red Sox first basemen in those years (with WAR and OPS+) compared to the Brewers:
1972: Danny Cater (1.0/85) v George Scott (4.9/124)
1977: George Scott (2.4/114) v Cecil Cooper (2.7/113)
1978: Scott (0.2/83) v Cooper (3.0/133)
1979: Bob Watson/Yaz/Scott (2.6/110) v Cooper (3.7/133)
1980: Tony Perez (0.6/108) v Cooper (6.8/155)
1981: Perez/Dave Stapleton (1.7/100) v Cooper (4.2/151)
1982: Stapleton (0.6/87) v Cooper (5.6/142)
1983: Stapleton (-1.6/76) v Cooper (4.0/138)
The first Scott deal looks somewhat better when you evaluate it in totality. That trade involved nine players but boiled down to Scott for outfielder Tommy Harper and starting pitcher Marty Pattin. In 1972, Pattin got off to a 1-7 start, then won 16 of his last 22 decisions and becoming a key part of the Sox's pennant run that ultimately fell a half game short of the Detroit Tigers. Harper had a respectable season but you can argue that the Sox might not feel that bad about the trade as of the end of 1972. Their real mistake was trading Sparky Lyle for Danny Cater to replace Scott.
In 1973 Pattin became an ineffective pitcher but Harper had an outstanding year playing center field, stealing 54 bases and accumulating 4.7 WAR. Pattin was gone after the season and Harper never had another good year for the Sox.
The trade that brought Scott back to Boston in December 1976 was a complete disaster. The Sox gave up Cecil Cooper to get Scott and Bernie Carbo. Bernie had been with Boston for the '74 and '75 seasons and was traded to the Brewers in June 1976 for Bobby Darwin and Tom Murphy. Darwin's slash line for the Sox that year was 179/216/349, walking twice and striking out 35 times in 106 at bats, one of the worst batting performances in baseball history. Murphy, a reliever, had a 6.75 ERA in 16 appearances.
While Cecil Cooper became a star, Scott had one decent year with the Sox, and then a season and a half of rapid decline before being dealt to Kansas City. He retired after the 1979 season. Carbo had an okay year in 1977 as a part-time player (2.3 WAR), before falling off rapidly in 1978 and being sold to the Cleveland Indians in mid-season.