I've been dealing with HELOCs off and on since 1977 when Commercial Credit Corp introduced them into their product line. In all these years I have never heard of anyone contemplating collecting "prepaid interest" on a HELOC.
My first thought it would be a violation of your loan contract since accrued interest is generally calculated on the average daily balance at the end of the billing cycle. At the opening of the account you have no average daily balance to calculate accrued interest on for a billing cycle.
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The opinions expressed are mine and they are not to be taken as legal advice.