Is it permissible to cover a deficit in the company's budget with the salary of a dismissed employee, and to distribute the surplus among the employees? And is donating this amount to help a friend with his marriage considered a valid way to dispose of it?

This increase is not rightfully yours or your manager's; rather, it belongs to the company owner. Both of you must repent to Allah and return the money to its owner, for the Prophet, peace and blessings be upon him, said: "On the hand is what it took until it returns it." Therefore, promptly return the amount you took and gave to your friend to the company treasury, and advise your manager and everyone who benefited to do the same and repent to Allah.