What should I do with money I found, whose owner I do not know, nor do I know those who used to work in the building? Is it permissible to take it out of the country, and on what should I spend it?

You were negligent in fulfilling the required identification and made a mistake by borrowing the money, thus becoming a guarantor for it through your transgression. You must complete the identification. If you find the owner of the money, it must be returned to him, unless you have given it away as charity and he has permitted you to do so.