Mark turned to the famous sculptor Titius

Mark turned to the famous sculptor Titius

Mark turned to the famous sculptor Titius with a request to sculpt a statue of Venus for him. Titius agreed to complete the work within a month. The parties agreed that in case of non-fulfillment of the order, Titius would pay a forfeit. A few days later, Titius died unexpectedly. Mark demanded that the heirs pay a forfeit.
Is this requirement legal?

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