What is the ruling on swearing not to demand repayment of a debt, and is expiation (kaffara) required? And does demanding repayment of a debt fall under Allah's saying: "So He made hypocrisy their consequence—in their hearts—until the Day they will meet Him—because they broke their promise to Allah and because they used to lie"?

Invoking Allah as a witness to an action or its absence is considered an oath if you intended it as such. The same applies to phrases like "Allah is my witness," "Allah bears witness for me," or "Allah is my guarantor." If you intended it as an oath, then you may take the money, and you owe an expiation for breaking an oath. The expiation for an oath is: freeing a slave, or feeding ten poor people, or clothing them. And for whoever cannot find [the means], then fasting for three days, as Allah the Almighty says: (Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten poor people from the average of that which you feed your [own] families, or clothing them, or the freeing of a slave. But whoever finds not [the means] - then a fast of three days. That is the expiation for oaths when you have sworn. And guard your oaths.) Al-Ma'idah/89.