Of course, cash it but don't spend it until you get a satisfactory answer.

I am not familiar with disability but have experience with government overpayment when in the military. It sometimes happens that the government gives you too much money for whatever reason. They always figure it out and want their money back sooner or later. There are many stories I heard about while in the army of people that received double pay and then spent it. Eventually the error was discovered and then the individual would not receive any money until the overpayment had been recouped. All the soldier receive wasn't earning and leave statement (ELS) that stated No Pay Due.

If they made a mistake they will expect it back.

Sound like it could be a possible change in designation from supplemental ( SSI ) to full disability (SSD). This happened to me during the time my dad was sick and on disability which meant I qualified for a change in status. But when my dad got better, and off help then my status reverted to just supplemental again.

If social security decides that you should have been eligible for the higher payment they will give out big lump sums or just switch from that point forward.

Though my advice before you know more is to continue sticking to the budget like the recipient only gets the smaller 500 payment if possible. Because it’s quite possible they will decide they have erred and then send a scary letter asking for the money back ( which is what happened to me )