Is it permissible to compensate for damages caused by delayed payment in sales contracts, such as the seller missing investment opportunities, provided there is a guarantee that is only acted upon after the actual occurrence of the damage and for its real value only?

It is not permissible to stipulate a penalty clause or a financial fine for late payment in a sale, because it is an interest-based condition. A penalty clause is permissible in financial contracts except for those in which the primary obligation is a debt, as this falls under explicit usury. This differs from a penalty clause in other rights and obligations—excluding debt—for compensation for actual damage; there is no harm in that.