To Settle Into
To Be Satisfied
intransitive verb, godan verb
If 納める is "to supply" or "to deliver," then this word is when things have been delivered: so it's to settle into or to be satisfied.
納まる means something or someone has found its right place and settled there. It often implies that there was a prior state of uncertainty or dissatisfaction, but now everything is in its right place.
Like most verbs with まる as okurigana, this is an intransitive verb. That means it happens by itself, with no emphasis on the person or thing that's making something settle or be satisfied.
The reading is the same as 納める.
金を払うだけで、彼らが納まるとは思えないけどな。
I don't think they will be satisfied with just money.