Mr. Tiwari,
At the risk of boring the others, I will point out the technical (legal) part of the matter here.
Under the Contract Act:
1. Sec 2 (h) - 'An agreement enforceable by law'.
2. Sec 2(b) - Promise: 'When the person to whom the proposal is made signifies his assent thereto, the proposal becomes an accepted proposal. A proposal when accepted becomes a promise. A promise to do or abstain from doing something becomes an agreement when it is accepted by all the parties involved in the agreement.
Essentials of a valid contract: Sec 10
* Two Parties - A valid contract must include a minimum of two parties; one that makes the offer and the other to whom the offer has been made and who must accept the proposal for it to become enforceable.
* Legal Obligation - The parties entering into a contract must have the intention of entering into a legal obligation.
* Certain Terms - A legal contract must have certainty of meaning.
* Possibility of Performance - A contract is considered valid only when it does not involve the performance of an impossible act.
* Free Consent
* Competency - The parties must be legally competent to enter into a contract.
* Consideration - The contract must involve consideration as per the principle of 'quid pro quo' or something in return.
* Legal Consideration - Section 23 of the Contract Act defines legal consideration as something not forbidden by law.
Now, please tell me which of the above is missing in an Offer Letter? It is a proposal made by one party, accepted by the other, done of free consent by persons competent to enter into an agreement. There is a consideration too (and the consideration need not be the exchange of money).
Actually, either side can sue the other in court for a breach of terms of the accepted offer. It is a general practice that an appointment letter is issued (legally required in a few states) when the person joins. If it is issued and accepted, it will override the offer letter. Till then, the terms of the offer letter are valid and enforceable.
If the candidate takes offer letters from multiple parties, none of them is enforceable at that stage, but once he accepts it, it is binding on him and the company. If he is accepting multiple of them, it is legally actionable by any or all of them.
With reference to the Notice Period, there is no notice period during probation only if it says so in the offer letter/appointment letter or the standing orders. Otherwise, the normal notice period applies. If you think no one needs to give notice during probation, that is a misconception. Most companies provide for it, but it is not the law.