Was it wrong of me to assume my builder would wave a 300 dollar bill after we aided his winning of 10k?
How do you think he would react to that?
If he still asks for it then he shall still receive it.
I've helped someone in the past that I think that they did not reciprocate when I needed them to, So what you're feeling right now is understandable. But assuming that your builder would wave a bill just because you helped him win $10,000 it's not something that you should do because they may not be aware that you needed them to waive it, and you might be disappointed in the process.
But why do I have to do that, I already know what will happen so it's easy for me.
I would recommend that you talk with him and see if there's still a need for you to pay that bill. Let him know that you will consider him for your future business and recommend your friends/family to him about being so accommodating with your payment.