Contingencies _ The buyer's offer is contingent upon certain keys facts being confirmed. One example may be confirming that the current lease is transferable to the new owner. Also, their willingness to buy is contingent upon the due diligence period confirming that everything you have stated about the business is true.
Both agreements are legal and binding documents and are subject to clauses and conditions of the document, when signed by both the buyer and seller. An offer to purchase is more common in small transactions and the letter of intent is more common in large transactions.