I don't see there where it says that the Holy Spirit's only function in the believer is to be a stamp demarcating that this one is guaranteed an inheritance. Seals do more than just mark ownership. They often also prevent intrusion by unauthorised persons or unwanted things. A sealed tomb, a sealed jar. a sealed letter.
Also, arrabon does not mean a deposit guaranteeing, but a down-payment, and earnest, indicating one's sincere intention to take full possession of something at a later date.When I put a deposit on a house, I am confirming my genuine desire to own the house. However, not all house deals go through, and often it is not the intended buyers fault when it fails.