Is it permissible to sell a website that was vowed to be established as an Islamic website for spreading Islam and for the conversion of Christians to Islam? And does the failure of Christians to convert through it, or its sale, entail an expiation by fasting?

Allah has commanded the fulfillment of vows, and it is obligatory in the manner in which the vow was made by its maker. It is not permissible to dispose of a property vowed to Allah except in a way that is more beneficial, because it has exited the ownership of the one who made the vow. Therefore, it is not permissible to sell it. However, if its management becomes difficult or impossible, it must be handed over to those who can manage it, whether they are individuals, associations, or official bodies. The one who made the vow is not responsible for anything after expending all efforts in guidance and دعوت if the invited ones do not respond, for the Sharia commands taking means, and the results are in Allah's hands.