yes, if a girl is too tired to deliver the service, she should just leave and not take your money.
But usually the little head takes control.... in this case, a little bit of humanity and understanding is called for.
I once called late at night a girl that I have seen many times. She was 'really tired'. So, I just laid her down and gave her a long good massage and without her doing anything, I used her for a quickie DIY.
From then on, whenever I met her, I got everything. Nothing was said or discussed. I showed her my understanding by my actions and she showed me her appreciation by her actions later.