@suzubelle: The way I understand it:
He probably exaggerated some details of the story, without ever telling any lie.
i.e. yes the twin were exiled, yes the parents were sick, yes the parents offered all their belongings to have him take care of the kids even as merchandise
But the frog probably rejected their initial request a little more coldly.
Or that he wasn't going to really look for 'someone good' to sell them to, just anyone who's willing to pay for them will do (which may or may not be 'someone good')
Him calling Dias guileless is probably for Dias saying he'll pay for the kids' expense until now even without any suggestion from the frog.
@Shuzuchi: She can detect lies as well, and hey the kids are 'blue'.
@xyzzy: Yeah, that's a lot of hoops to get him to adopt the children, and to be fair he was going to refuse until Aruna stepped in.