Does demanding the thousand riyals after having called upon Allah as a witness that one would not take them fall under His, the Almighty's, saying: "So He put hypocrisy in their hearts until the Day they meet Him, because they broke their promise to Allah and because they lied"? And is expiation obligatory if the money is taken?

If someone forfeits their right to another by way of absolution and an intention to forgo, then they are not permitted to retract. However, if there was no intention of absolution, then they are permitted to retract and claim. The statement, "My Lord is my witness, I will not take it," is an affirmation of not taking. If this statement, or "this money is forbidden for me," is intended as an oath, then you may retract and claim, and an expiation for breaking an oath is required of you. This action does not fall under the threat mentioned in the verses of Surah At-Tawbah, because the verse speaks of those who make a vow to Allah and then are stingy and do not fulfill it. If your intention by making the money unlawful for yourself was to forbid it, then an expiation for breaking an oath is required of you if you take it. If it was a supplication, then there is nothing upon you.