In Roman law, a valid contract of sale required the parties to agree on a certain price. Some modern works nevertheless accept that the law ignored a certain species of error: the buyer is willing to pay more than the seller expects to receive, and a valid contract of sale is formed on the lower price. This supposed exception is based, not any text on sale, but on a single text on contracts of hire, Digest 19.2.52. This text suggests that, in some cases, a contract of hire might arise where the tenant believes he is paying a higher rent than the lessor believes. This text, however, turns on certain misunderstandings that would arise only in hire (e.g., a misunderstanding over the term of the lease). It therefore gives no guidance on whether a buyer who is willing to pay a higher sale price is treated like the analogous tenant.
Available at: http://works.bepress.com/ernestmetzger/6/