Is it permissible for a Muslim fiancé, whose work finishes after Iftar (breaking of fast), to break his fast at his fiancée's house and then leave, without violating Islamic rulings?

The suitor is a stranger to the fiancée as long as the legal marriage contract has not been concluded. Therefore, he is not permitted to be alone with her or to engage in excessive conversation with her without need. If your interaction with the suitor in your home is conducted according to Islamic guidelines, there is no objection to it; rather, helping him with his fasting and breaking his fast are commendable actions. It is preferable to expedite the conclusion of the marriage contract between you two, to remove any suspicion and to eliminate awkwardness in dealing with him, while it remains possible to postpone other marriage-related matters.