The 9% discounts the eventual acquisition.
It's used car tactics at the next level.
If he already thinks the business's days are numbered, options are limited. Instead of taking a loss by holding it until bankruptcy, or selling it cheap to someone who also sees limited value, he sells it to someone who's (overly) optimistic, and and is willing to pay a higher value.
Since the risk (at that higher value) is high, lenders will either refuse to finance, or offer a rate higher than what Buffet is offering. Buffet needs to offer the financing to make the sale possible, or at least more attractive.
So at worst, he's turned a dead-end investment into positive cash flow for a few months, where he previously was getting zero. If he forecloses and there's something of value left, he might try to sell again, but the foreclosure makes that less likely, given future buyers will consider it in determining it's value. Regardless, he's still better off than before.
Or the business does ok, and he gets a better price for selling than he otherwise would have. And 9% interest on it, which is something, but I suspect extremely low for Buffet. But he's locked in a minimum value and getting a positive rate, mitigating what he perceived to otherwise be a bigger loss.
A third possibility is that Lee has synergies that Berkshire doesn't have e.g. Lee can give it scale.