you know, Shmi's unlikely, unsatisfactory 'explanation' to Qui-Gon in TPM could also be interpreted as a complete 'lie' to Qui-Gon when he has the audacity to ask about it, lol. At the end of the day, we don't get to know what 'kind' of slavery she's involved in, and maybe she didn't 'know' who Anakin's dad was... And although there's been some comments about how a 'protocol' droid seemed an unlikely choice to help her out...it would be perfect for interpretive dealings with various alien unsavoury types on her behalf/Watto's behalf, if you get my meaning... No, no, surely not...)
Would explain why Watto was much more willing to give up the boy. His mom was the money maker. Otherwise you'd think he'd rather keep the younger one who would live longer, become physically stronger and therefore do more work.