Is it permissible to perform Umrah with money that includes a portion belonging to others without their knowledge, intending to repay it and sincerely repent in the Sacred House of God?

It is not permissible to take someone else's money without their permission, even if you intend to return it, and doing so is a sin. If you perform Umrah with money some of which is unlawful, the majority of jurists hold that the Umrah is valid, though sinful. Umrah is obligatory for those who are financially and physically able, and it is not considered a necessity that permits taking other people's money. Therefore, you must stop taking this money and performing Umrah with it unless you have the owner's permission, for the intention to repay does not justify the unlawful. Fear Allah and be patient until you are able to acquire lawful money for Umrah. Sincere repentance is accepted, by Allah's permission, in any place.