Have I committed a sin by keeping the engagement ring that the suitor gave to my daughter, in order to compensate for the amount I paid him to buy furniture?

Your payment of the agreed-upon amount between you and the suitor for the purchase of furniture for the two rooms is considered an agency. Therefore, your right is to receive the furniture, and you do not have the right to demand its price from him, unless he agrees to purchase the furniture from you. What you mentioned, that the furniture is only suitable for his apartment and you have no place to store it, does not affect your ownership of the furniture.