Ulpianus 4 ad ed.
Sed et si in alium contractum res non transeat, subsit tamen causa, eleganter Aristo Celso respondit esse obligationem. ut puta dedi tibi rem ut mihi aliam dares, dedi ut aliquid facias: hoc sunallagma esse et hinc nasci civilem obligationem. Et ideo puto recte Iulianum a Mauriciano reprehensum in hoc: dedi tibi Stichum, ut Pamphilum manumittas: manumisisti: evictus est Stichus. Iulianus scribit in factum actionem a praetore dandam: ille ait civilem incerti actionem, id est praescriptis verbis sufficere: esse enim contractum, quod Aristo sunallagma dicit, unde haec nascitur actio.
|But even if the matter does not fall under the head of another contract and yet a ground exists, Aristo in an apt reply to Celsus states that there is an obligation. Where, for example, I gave a thing to you so that you may give me another to me, or I gave so that you may do something, this is, Aristo says, a synallagma and hence a civil obligation arises. And, therefore, I think that Julian was rightly reproved by Mauricianus in the following case: I gave Stichus to you so that you would manumit Pamphilus. You have manumitted. Stichus is then acquired by a third party with a better title. Julian writes that an actio in factum is to be given by the praetor. But Mauricianus says that a civil action for an uncertain amount, that is praescriptis verbis, is available. For the contract described by Aristo with the word synallagma has been made and hence this action arises.|