Nina Robinson said:
in the state of maine can one party charge interest on a debt that the other one owes to them?
My response:
"Well, here's to you, Mrs. Robinson . . . "
Only if it's in the original contract, and the terms were negotiated between the parties prior to signing the agreement.
If there was no written agreement concerning interest, then no, you cannot charge interest "after the fact".
Are there more facts about your situation ?
IAAL