I can find only very little information on the abolition of slavery in the Kingdom of Hungary. The Wikipedia Timeline of abolition of slavery and serfdom only mentions that in 1958 Hungary ratified the 1926 "Convention to Suppress the Slave Trade and Slavery". This is not the same as abolishing slavery.
The lack of information on the subject suggests that slavery was not really a thing in the KOH.
On the other hand, the review of Slavery in Early Medieval Hungary suggests that:
The use of slave labour in Hungary continued until the end of the thirteenth century when a combination of economic and political factors brought it to an end.
However, I find it unlikely that this was the definitive end of slavery, since it is more than 400 years ahead of the other European countries, and the book deals only with "Early medieval Hungary".
Furthermore, this article suggests a revival of slavery after the black death in Europe.
Was slavery legally reintroduced? If so, when was it finally abolished?