There's an HR issue, in that you should probably start shopping for a new loan officer...
There's possibly a contract law issue - I'll wager that your installment loan contract doesn't allow for future advances. Often, that's a specialized document that isn't used in general consumer lending (apart from open end credit lines). My point is, once the customer satisfies the original note, he doesn't have to pay any more. The new money isn't covered under the typical installment loan contract. So basically, the customer just received a gift. And you've inherited an unsafe / unsound loan.
The big question in my mind is, why? Is there some connection between customer and lender? If so, then you're talking about a code of ethics breach ("arms-length transaction") and strong consideration of a SAR.
I don't know enough about the accounting issues to comment on how this transaction would be handled by your CFO. You probably also made changes to the customer's payment book that aren't supported by loan documentation. Based on the limited info you provided, I think you have a lot to sort out.
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Opinions are Bartman's, not those of my employer. "A noble spirit embiggens the smallest man."