This is the 4743rd most frequent Russian word.
заключенного
"Заключенного" means "prisoner" or "convict" in English.
Here, 'заключенному' is the dative case of 'заключенный' and refers to a person who is imprisoned, indicating to whom the action is directed.
Он задал вопрос заключенному.
He asked the prisoner a question.
In this context, 'заключенного' is the singular genitive form of 'заключённый', functioning as an adjective agreeing with 'контракта', meaning 'concluded' or 'finalized'.
Мы обсуждали содержание заключенного контракта.
We discussed the content of the concluded contract.
Here, 'заключённого' is the singular genitive form of the noun 'заключённый', referring to a detained person, fitting into the sentence as the one who could not be transferred.