Thanks so much Alex Mkrtchyan :)

but I disagree with you.

I'm sure that there are a lot of native speakers who like to help others without caring about money.

and I think it's not bad thing that they are tring to gain money from teaching here.

 I think native speakers are not active and its seemed to me that many of them wants to earn money here. Sorry but it's my opinion.

Thanks Gillian :)

Yes above board is used often.

My boss thought money I had stolen some money. I counted the money so that he could see that everything was above board. (open and honest)

That teacher is above board. (he is honest)

Is that place we are going to above board? (legitimate)

Farnsworth Shepard said he was well aware of the Farragher group's history and said the liquidation would be above board. (honest and legitimate)

“I thought for his employment to be verifiable it has to be above board,” the police chief said.

In response to your original question what I would most naturally say is: 

A : I didn't forget our appointment. It was a very tough day for me. I feel so tired. But I will not miss our appointment.
B : It's ok. If you can't come today, I understand. 

The next choice would be "I would understand."

I'm afraid that I can't tell you why grammatically as I'm not an English teacher, just a native speaker spending my time to help you here for free (please take note of that Alex). ;D