What is the ruling on supplying public institutions with cleaning materials, then purchasing other needs for them or giving them a sum of money that will be calculated later within the cleaning materials invoice, knowing that this is a common practice in Algeria?

Institutions are not permitted to alter the terms of expenditure unless authorized to do so. If authorization exists, then there is no objection to cooperating with them and selling materials to them at a profit. However, directly lending money with interest is prohibited usury (riba). If the higher authorities do not permit the change, then it is not permissible to cooperate with them, as this involves deceit and a violation of Allah's saying: "And do not cooperate in sin and aggression," and the Prophet's (peace be upon him) saying: "He who cheats us is not of us." Verification of authorization from the responsible authorities is necessary.