Am I obligated to return the price of the dress to my friend who bought it then returned it, claiming she didn't like it, even though I don't have the money now, and the sale has been finalized and the parties have separated?

If the dress sale contract has met the conditions and is free of impediments, and you have parted ways from the contract session, and no hidden defect was discovered, then the sale is binding. The dress belongs to the buyer and the price to the seller. The seller is not obliged to return the money. It is, however, commendable in Islam to annul a sale for someone who regrets it.