How to word prepayment penalty when borrowing

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I've negotiated a purchase with 10% down and seller carry back for the rest. Seller agreed to purchase price and terms. Loan is for 120 days and the seller wants to be guaranteed the 120 days of interest (6%) regardless of payoff date. Can anybody help me on the wording for the note? It's too late to ask my escrow officer and I need to get this out asap. Is that called a prepayment penalty or is there some other way to word the guaranteed interest?