What is the ruling on disposing of surplus funds, for which there are no documents, that resulted from a method of calculating the average currency exchange rate in a company whose manager is aware of this, given the employee's fear that the company might demand them back in the future?

Your husband is an agent for the company, and an agent does not act except with the permission of his principal. Therefore, he is not permitted to own the excess money he has unless authorized by the company owners. If they gift it to him, there is no problem. But if they do not gift it to him, he must return it to the company.

The way to ascertain this is by asking the company officials about how to dispose of excess money. It is not permissible to burden an employee with deficits in accounts unless there was a transgression or negligence on his part.