The East Ilsley-based trainer could have faced a lengthy ban as a consequence of Our Little Sister's positive test for nandrolone following a race at Wolverhampton on January 14 2017.
However, an independent disciplinary panel of the British Horseracing Authority found that the banned substance was administered intentionally "by person or persons unknown, for unknown reasons" and gave Morrison the minimum fine of £1,000.
The 57-year-old trainer accepted he was technically in breach of the BHA's strict liability anti-doping rules as a result of the positive test, but always denied any wrongdoing.
Our Little Sister had finished last of eight runners in an extended two-mile handicap, in which she was sent off at odds of 12/1.
She raced once more, when down the field at Southwell 12 days later, and has since been retired.