You had accepted the appointment letter and joined the organization, and you were there for two days as well. Although you have not signed any other documents, you are bound by the appointment letter which you have acknowledged. The terms of the appointment letter alone can clarify your liability. The fact that you were there for only two days does not exempt you from this liability.
If the organization is willing to release you without paying you any sum for your two days' attendance, that is the best alternative for you. You may try to persuade them to accept this proposal. According to the provisions of the Contract Act 1872, the organization can only charge for the breach of employment terms, i.e., the direct expenses incurred for you. So even if they resort to legal remedy, there is not much they can claim from you.