Does a person have to pay a sum of money agreed to in a divorce property settlement?
Can they just say I don't have the money to pay it?
They need to honor their Agreement. Person would be subject to contempt for non-payment or penalized for fraud.
They are obligated to pay. A money judgment can be entered against them and effect credit rating. Work out a payment schedule.
If they do not pay, it is a court order so you can file a motion for contempt/enforcement.