Theses de emptione et venditione

X.
Just as there must be a certain thing, so there must be a certain price, either from the beginning or determined after the fact: it must not, however, be left to the judgment of either contracting party.

XI.
Likewise, it must consist of counted money. For in order for buying and selling to be contracted, a body must be exchanged with a quantity: so that, if body is not exchanged with body, or quantity with quantity, a nameless contract is not made—that is, a permutation exchange or barter in the general sense.

XII.
However, the giving of arra earnest money/deposit is not required for the perfection of buying and selling. For what intervenes through the giving of earnest money does not create the obligation, but demonstrates the one that exists: indeed, it sometimes constitutes a part of the price.

XIII.
And indeed, if earnest money has been given as evidence of the buying and selling, there is a place for repentance: yet in such a way that, if it is the buyer who refuses to fulfill the contract, he loses what he gave: but if it is the seller, he must restore double. It is otherwise if the earnest money is imputed toward the price.

XIV.
The effect of this contract is that, once it is perfected, if