Is selling unlimited storage space (based on the "fair use" principle and promotional advertising), while knowing that there is no actual unlimited storage, permissible or is it dubious?

It is permissible to rent known spaces on servers and then lease them to website owners. However, it is not permissible to rent open, undefined spaces due to the prohibition of the Messenger of Allah (peace be upon him) on gharar (deception/uncertainty) in sales, which refers to ignorance and risk-taking that leads to disputes. It is a condition for the subject of the contract to be specific and known to both parties to distinguish it from others and eliminate excessive ambiguity.