In The Phantom Menace, after Anakin wins the race, they sell the pod and give the money to Shmi. In Watto's words: "No pod is worth two slaves", however, clearly a pod is worth at least one slave given that Watto betted Anakin.
After the race Watto also says that he "lost everything", therefore, one would assume that he would be more inclined to sell Shmi to get back some money.
Plus, they still have the 20,000 Republic Dataries, which, assuming have the same value as Imperial Credits, could almost buy two ships according to Luke. And from the Original Trilogy we also know that at least some people in Mos Eisley accept Imperial Credits, making it possible to exchange the Republic Credits to something that has value in Mos Eisley.