Is it permissible to use financial surpluses resulting from employers' injustice towards employees, or from mistakes made with customers, to cover deficits in the fund or to cover personal expenses, knowing that employers do not tolerate any deficit?

The accountant is a private employee, and his hand over the institution's funds is a hand of trust. Any surplus in the cash box must be returned to the employers, for Allah Almighty says: ﴿Indeed, Allah commands you to render trusts to whom they are due﴾. And in the Hadith (definition): "The hand is responsible for what it takes until it returns it." The injustice of employers is not a justification for disposing of their money, and what is obligatory is to advise them, for the Prophet ﷺ said: "The religion is naseeha (sincere advice)."