Property Management: An employee mishandled a property

[Robert Thomas]

Employee took over a client's account when the agreed lease was $1000 , for some reason the tenant had been sending in $1100. Employee did not ask about this discrepancy and changed the account to $1100 without having tenant sign new lease. Landlord has been receiving profits under $1100 figure. Today tenant pulls out current lease ($1000) and demands the overpaid amount back. What are my options?