Chce spłacić pieniądze które jest dłużny. (Sentence Note)

  • dłużny (adj) = indebted, owed
  • dłużnik (mp noun) = debtor
English Translation

He wants to pay back the money he owes.

Owing someone something or being owed is something I always struggle with in Polish. Why is it “które jest dłużny” here? Shouldnt the adjective be in accordance to które? And if thats not the case, shouldnt a specific be used in the adjective?