SUPERVENING FACTS COUNTERMAND ALL LAW
The Fugitive Slave Act of 1850 (FSA) was not repealed by the Emancipation Proclamation of 1863. FSA was not repealed until June 1864. For 1 and 1/2 following January 1, 1863, most black Union soldiers "recruited" under the Proclamation were subject to being returned under law to masters. Some actually were!
However, practicalities sometimes temper the stridency of law. Thus in May 1861, three slaves who had rowed over to Fort Monroe, Virginia, near Jamestown, presented themselves to its commanding officer, Gen. Benjamin F. Butler, a lawyer. Butler refused to surrender them to their Confederate commander for whom they had been consigned to dig trenches and artillery fortifications against Ft. Monroe. Being a reasonable man, and no fool, Gen. Butler refused to return the three heroic, daring slaves whom he deemed "contraband of war." They were already deemed to be "property" like grain or livestock captured in battle, he reasoned. Abraham Lincoln, a lawyer, concurred. So, the FSA law became a dead letter by default, overcome by supervening facts!
"The demand from the South for more effective legislation resulted in enactment of a second Fugitive Slave Act in 1850. Under this law fugitives could not testify on their own behalf, nor were they permitted a trial by jury. Heavy penalties were imposed upon federal marshals who refused to enforce the law or from whom a fugitive escaped; penalties were also imposed on individuals who helped slaves to escape. Finally, under the 1850 act, special commissioners were to have concurrent jurisdiction with the U.S. courts in enforcing the law. The severity of the 1850 measure led to abuses and defeated its purpose. The number of abolitionists increased, the operations of the Underground Railroad became more efficient, and new personal-liberty laws were enacted in many Northern states. These state laws were among the grievances officially referred to by South Carolina in December 1860 as justification for its secession from the Union. Attempts to carry into effect the law of 1850 aroused much bitterness and probably had as much to do with inciting sectional hostility as did the controversy over slavery in the territories.
"For some time during the American Civil War, the Fugitive Slave Acts were considered to still hold in the case of blacks fleeing from masters in border states that were loyal to the Union government. It was not until June 28, 1864, that the acts were repealed." https://www.britannica.com/event/Fugitive-Slave-Acts