The right to rescind or modify a contract of sale, the terms of which have previously been arranged, is always open to the parties whether the contract is executed, or executory merely. Mere abandonment of the sale, by mutual agreement, is sufficient to discharge an executory contract. A contract executed as to one of the parties only, can only be discharged by performance by the other party, or by release under seal, or by an agreement supported by a proper consideration to release the party who has not performed his part of the agreement. It is competent however, for the parties to substitute a new agreement in place of the original contract; the general rules as to the right to rescind the contract apply to the right to rescind in a contract of sale.