What is the ruling on working in programming applications for download centers or blogs, with the condition that they are not used in ways that violate Islamic law, if it is highly probable that customers will download prohibited materials on them?

The default ruling for creating shared programs and systems that might be used for both lawful and unlawful purposes is permissibility, unless it is highly probable that they will be used for unlawful purposes, in which case their creation becomes impermissible.

If one knows or strongly believes that they will adhere to the condition of not using the program in anything that violates Islamic law, then there is no objection to creating it. However, if one knows or strongly believes the opposite, then it is not permissible to create it. If one does not have a strong belief either way, the default is permissibility, and their agreement to his condition is sufficient at that point.