Arbun is an earnest money deposit or down payment in Islamic commercial transactions. The buyer pays a portion of the price upfront to reserve the right to complete the purchase later. If the buyer proceeds, the deposit counts toward the total price. If they back out, the seller keeps the deposit.
Think of it like booking a hotel room with a non-refundable deposit. You pay $100 upfront for a $500 room. Show up, and you only owe $400 more. Cancel, and you lose the $100.
In Islamic finance, Arbun is used in Islamic options and forward contracts. It gives the buyer flexibility without the full commitment. The Hanbali school of Islamic jurisprudence permits Arbun, while other schools have historically debated its validity.