Gaius, On Verbal Obligations, Book I. Loans of money are frequently made for the purpose of enabling them to take the place of payment.

Gaius, On Oral Obligations, Book I. If I stipulate conditionally with a principal debtor, I can bind a surety for both this condition and another, provided I unite them; for, unless both of them should be fulfilled, he will not be liable, as the principal debtor is bound by one condition alone. If, however, I separate them, the condition of the surety will become more onerous, and on this account he will not be liable; because, whether a condition will affect both of the parties bound, or only one of them, it will be considered to hold him; while the principal debtor will not be liable unless the common condition is fulfilled. Therefore, either the surety will not be liable at all, or, which is the better opinion, he will be liable if the common condition is previously fulfilled. 1When sureties are interrogated under different conditions, it is a matter of importance to ascertain which one was first complied with. If it was the one imposed upon the principal debtor, the surety will also be liable when this condition is fulfilled, just as if from the very beginning the principal debtor had been absolutely bound, and the surety had been bound under a condition. On the other hand, however, if the condition of the surety should first be complied with, he will not be liable, just as if he had been absolutely bound from the beginning, and the principal debtor was only bound conditionally. 2When the principal debtor is liable for a tract of land, and the surety is accepted for the usufruct, the question arises whether the surety is liable to a less extent, or, indeed, whether he is liable at all, as having promised something else. It does seem to us to be doubtful whether the usufruct is a part of the property, or something which exists by itself. But as the usufruct is a right attaching to the land, it would be contrary to the Civil Law for the surety not to be bound by his promise. 3A surety can be accepted by a slave, just as his master, himself, can legally accept one for the amount due to him; and there is no reason why the surety should not be interrogated by the slave himself. 4If you should stipulate with an insane person, it is certain that you cannot take a surety; for not only is the stipulation itself void, but no business at all is understood to have been transacted. If, however, I should accept a surety for an insane person, who is liable by law, the surety will also be liable. 5When it is commonly asserted that a surety cannot be received for criminal offences, it should not be understood that anyone who has been robbed cannot take a surety for the payment of the penalty for theft, as there is a good reason that penalties incurred by crimes should be paid; but rather in the sense that a person cannot bind the surety for part of the proceeds of a theft, which he desires to be given to him by someone with whom he committed the offence; or where, by the advice of another, he was induced to perpetrate a theft, he cannot take a surety from him with reference to the penalty for the crime. In these instances, the surety does not become liable, because he is not furnished in a valid transaction, and partnership in an illegal act is of no force or effect.

Gaius, On Verbal Obligations, Book I. The statement, “Which are worth more than thirty aurei,” has reference both to a sum of money, and the valuation of property.