[PAULUS libro secundo ad edictum. ] §45.1.68.prSi poenam stipulatus fuero, si mihi pecuniam non credidisses, certa est et utilis stipulatio.
[PAULUS, On the Edict, Book 2] If I have stipulated for a penalty, in case you should not lend me money, the stipulation is certain and valid.
quod si ita stipulatus fuero: 'pecuniam te mihi crediturum spondes?', incerta est stipulatio, quia id uenit in stipulationem, quod mea interest.
But if I have stipulated as follows: 'Do you promise that you will lend me money?', the stipulation is uncertain, because that which is my interest comes into the stipulation.