Is this sermon considered a promise, and is the suitor sinful if he does not return to complete the marriage after giving the fiancée the choice between coming with him or waiting for a specified time and then becoming free?

It is understood from the question that the inquirer did not make a firm promise of marriage to the girl, and if he does not return after the Eid, she is free. Fulfilling a promise is legally recommended and is one of the noble virtues, while breaking it is a characteristic of hypocrites. However, not fulfilling it does not entail sin. It should be noted that conversing with non-mahram (definition) women is not permissible except out of necessity, and with clear guidelines such as avoiding seclusion and softening of speech.