Is it permissible to continue working with a project supervisor who requests the purchase of sand for 60 dinars, then declares to the project owners that its price is 120 dinars?

It is not permissible for a project supervisor—if he is an agent for the owners—to buy sand at one price and then inform them of a higher price, because that is a lie and consuming people's wealth unlawfully. In such a case, it is not permissible to continue working with him, as it constitutes cooperation in sin. However, if the supervisor buys sand and sells it to the project owners at a higher price with their consent, and he is not their agent, then there is no harm in that, nor in continuing to work with him.