I have what this phrase (dedere se in fidem) means, but it is here necessary to remind my readers briefly of its significance.
Those who thus commit themselves to the faith of <a class="linkToPlace" target="_blank" href="/place?placename=Rome&groupId=935&placeId=1669">Rome</a> surrender in the first place the whole of their territory and the cities in it, next all the inhabitants of the land and the towns, male and female, likewise all rivers, harbours, temples, tombs, so that the result is that the Romans enter into possession of everything and those who surrender remain in possession of absolutely nothing.
Shortly after this surrender had been made by the Carthaginians they were called into the senate, where the praetor conveyed to them the decision of the senate, that as they had been well advised, the senate granted freedom and their laws, besides their whole territory and all other possessions both public and private.
The Carthaginians on hearing this were pleased, thinking that in the choice of evils they had been well treated by the senate, as all that was most essential and important had been conceded to them.
But after this, when the praetor informed them that they would obtain these favours if within thirty days they sent to <a class="linkToPlace" target="_blank" href="/place?placename=Lilybaeum&groupId=690&placeId=1261">Lilybaeum</a> three hundred hostages, sons of senators or of members of the Gerousia, and if they obeyed the orders of the consuls, they were somewhat at a loss to know what these orders would be. However they left at once to announce all this at <a class="linkToPlace" target="_blank" href="/place?placename=Carthage&groupId=441&placeId=820">Carthage</a>, and on coming there they informed their fellow-citizens of all the particulars.
On hearing these they all thought that the envoys had obtained a satisfactory result in general, but the fact that there was no mention of the city caused serious anxiety and surprise.
Walbank Commentary