If the first offer had not yet been accepted, the seller could have chosen instead your back-up cash offer. But you write that the seller accepted the other offer and its contingencies have been met; in that case, unless the buyer(s) breach or violate the contract in some material or important way (like missing some payment or being unable to close), the seller is contractually obligated to other offer and cannot sell to you. If there is a breach by the other buyers, that will let the seller terminate the contract if he/she chooses.