Celsus, Digest, Book XVIII. If anyone who is transacting the business of a surety, paid the stipulator with the understanding that he would release the debtor and the surety from liability, and he does this in compliance with law, he can hold the surety liable in an action on the ground of voluntary agency; nor does it make any difference whether or not the surety has ratified his act, for he will, nevertheless, be entitled to an action on mandate, as soon as he does ratify it, and even before he pays the money to the agent. 1Where a certain amount of grain is due, and the surety delivers African grain, or, impelled by the necessity of payment, he gives something of greater value than the price of the property to be delivered, or he transfers Stichus, and the latter dies, or becomes worthless either through weakness or vice; the amount can be recovered by an action of mandate.

Celsus, Digest, Book XXXVIII. Whatever is required to render an obligation binding is understood to have been omitted, if it is not plainly expressed in words; and we almost always interpret it in favor of the promisor, because the stipulator was free to give a broader meaning to the terms; but, on the other hand, the promisor should not be heard if it is to his interest that the agreement should be considered to have reference to certain vessels, or to certain slaves. 1If I stipulate as follows, “Do you promise to pay if you do not ascend to the Capitol within two years?” I cannot legally bring suit before the expiration of the two years; for although these words are ambiguous, still they are understood to have this meaning, “If it is absolutely true that you did not ascend to the Capitol.”

Celsus, Digest, Book XXXVIII. Almost all the rights of heirs are considered to be the same as if they had become such immediately after the death of the testator.