Marshall v. P. R. Beverly was a case that came before the United States Supreme Court in 1820, and it concerned the issue of whether or not slaves were considered property under state law. The plaintiff, Marshall, had purchased two slaves from defendant P.R. Beverly but later found out that they had been previously promised to another person by their former owner as part of an inheritance dispute between siblings over their father's estate; thus making them unavailable for sale at the time of purchase by Marshall without knowledge of this prior agreement on behalf of his seller (Beverly). In its ruling, the court held that although slavery was recognized as legal within certain states according to local laws and customs at this time period in history, these same laws did not grant any rights to slave owners regarding ownership or transferability; instead such matters were subject only to common law principles which could be enforced through civil action if necessary - meaning that even though Marshall may have legally acquired title over these individuals when he bought them from Beverly initially - he still lacked sufficient evidence proving his right against other claimants who also claimed ownership due to pre-existing agreements made with previous owners/sellers prior thereto – thus rendering him unable to maintain possession unless otherwise proven so via proper judicial proceedings outside those related solely towards enforcing state statutes concerning slavery itself (which would have no bearing here).

In Marshall v. P. R. Beverly, the Supreme Court was asked to decide whether a Virginia statute that allowed slaves to be emancipated by their owners was constitutional under the United States Constitution and applicable federal law. The majority of justices held that it was not, as Congress had exclusive authority over slavery in all states and territories within its jurisdiction; thus, any state laws allowing emancipation were unconstitutional because they interfered with congressional power over slavery. Justice Johnson dissented from this opinion on two grounds: firstly, he argued that since there is no express prohibition against such state legislation in either the Constitution or federal statutes, then it should be presumed valid until proven otherwise; secondly, he maintained that even if Congress did have exclusive authority over slavery matters within its jurisdiction (which he doubted), then this would only apply to new cases of emancipation rather than those already existing before the passage of any relevant Congressional acts – which applied here since Virginia's statute predated any relevant Congressional action concerning emancipation.