A friend is moving into a new apartment. The contract defines that every month my friend pays

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Question:

A friend is moving into a new apartment. The contract defines that every month my friend pays rent of 500 to Rent Spa. If my friend buys the apartment at the end of October 2023, he will see all the rents paid deducted from the agreed price of 2000 per m2. What type of financial instrument is linked to this scenario