Collateral Agreement Rule
The second case in which evidence from Parol is admissible is evidence of ancillary agreements. One theory says that it is possible to accredit as a guarantee contract for a third party, since credits are motivated by the buyer`s necessity and, in application of Jean Domat`s theory, the cause of a credit is that a bank issues a credit in favor of a seller in order to relieve the buyer of his obligation to pay directly to the legal tender seller. There are indeed three different entities that participate in the accrediting transaction: the seller, the buyer and the banker. Therefore, a credit corresponds theoretically to a guarantee contract accepted by the conduct, or, in other words, to an implied contract.  It is briefly called loc The rule of parol proof is everything related to external evidence and contracts. When a contract is “integrated” and concluded, a party will find it difficult to introduce external evidence of other agreements or commitments. However, there are many exceptions that sometimes allow external evidence to be introduced. The second convention was outside the evidence, but a court allowed their introduction for two reasons. First, the oral agreement did not contradict the written and fully integrated option agreement. Second, a commission agreement is not something that similar parties could normally include in a real estate purchase agreement.
Ancillary contracts are an exception to the legal doctrine of the Treaty which provides that a contract may not impose obligations or confer rights on a non-contractual party.  However, in cases where an ancillary contract is entered into between a third party and one of the contracting parties, the Court may assert rights or impose obligations on the non-contracting party, as shown in the earlier Donoghue case against Stevenson.  A party to an existing contract may attempt to demonstrate that there is an ancillary contract in the event of a failure of its right to the infringement, because the statement on which it relied was not considered a term of the main contract. It was found that, for this to be successful, the statement had to be guilty.  Remedies may be granted in the event of a breach of an ancillary contract. Consider De Lassalle v. Guildford, a case of a secondary contract in which the latter party rented a house to the former. The landlord promised to fix the flow before the tenant moved in.
 It was decided that, even if the main order does not concern the auction, the advantages granted to the auction for the increase in the price of the offer constitute a good consideration.  This rule prevents the parties from altering the importance of written contracts by oral or tacit agreements that are not included in the original contract and thereby undermining its integrity. In other words, if a contract is concluded in writing, subsequent agreements that are not concluded in writing are not used as evidence in a contractual dispute. . . .