I have been given money by my boss to use for office expenses, and in his absence I placed the unspent amount remaining on his office table in a sealed envelope. I later found out the envelope was stolen and my boss never received his money. Must I refund him the stolen amount (about Rs1800)?

Know that the money is an amaanah or a trust in your hand. You are an ameen or a trustee. If the trustee faults in keeping the amaanah in a safe place, then he would be liable for the lost. But if he kept it in a safe place and still got stolen, then he would not be liable. So the answer is to you whether that office was safe place.