Offer your insights or judgment on the input financial query or topic using your financial expertise. Reply as normal question answering

Pay index fund expense ratios with cash instead of fund balance
Simply put, that's not allowed.   Outside a retirement fund, they simply do not provide a mechanism to pay that expense ratio separately.  Ergo, any effort to pay that expense ratio would be classified as a new/additional purchase of the fund. You now must deal with  Inside a retirement fund, paying the expense ratio of the fund with cash would be treated as an additional contribution, which may then violate contribution rules (such as going over your contribution limit, or contributing past age 70-1/2).