I can see the problem from both sides but really Heroku had some more more to gain by just crediting it or most of it in what you would say about them. Clearly by the rules, you should have to pay, but that does not mean that a business should not do the right thing and work with a customer.

Imagine how big a raving fan you would have been if they had credited the entire thing. That is a tiny cost compared to acquiring a new customer and turning them into a raving fan.