Is it necessary to scrutinize the source of funds of an entity offering a bank guarantee for an investment project that is permissible in Islamic law, or is it sufficient to accept their apparent dealings, and what is the ruling if a Sharia-related doubt is later discovered concerning these funds?

Unless there is strong evidence or an indication of the unlawfulness of the money from this guarantee, there is no need to investigate its source, as the default assumption for a Muslim's wealth is permissibility. Therefore, there is no objection to accepting the guarantee provided by this entity if it adheres to the general Sharia guidelines. However, if it later becomes clear that the source of these funds is unlawful, then it is not permissible to deal with them.