I've thought awhile before posting because I have no experience using a known donor to conceive. Also, not knowing the dynamics between you and your friend makes clear sight difficult. Still, I had a few thoughts come to mind, and thought I would share them.
A couple of questions first:
1. Is your friend holding his help for you over your head in any way? Using it to get his way? ("Since I am doing this for you I really want you to do for me.")
2. Is he outwardly willing, but are you sensing any angsty passive aggressive tendencies from him? ("Oh sure, I can be available the weekend of November 7th. i mean, I was kinda hoping to go on a trip with a few friends, but I guess I can stick around for you, if that is what you really need me to do. Sigh....")
I think what i am trying to ask is that do you feel he is internationally trying to use this perceived power over you?
If the answer is yes then this situation might not be the best for either of you, and you might want to consider other donor options.
I can actually see where your best friend is coming from here. If I were a donor to someone close to me, if I were giving from my heart and not because I had an agenda to push, it would be really hard for me to understand my good friend friend viewing the situation as me having a position of power over them. In fact, if that were the case it would feel like a no-win situation for me. If I don't help my friend I know i am letting them down in a big way and disappointing them. If I do help my friend I am setting myself up to be a threat in their mind, a proverbial ax hanging over their head. Unless everything went perfectly and quickly I would fear the friendship might be forever damaged. As the friend doing the favor this would be a scary position for me.
In my mind many perceptions of power are just that: perceptions. It all depends on how you look at certain situations.
To take an example away from baby-making.
Here is how my partner and I could view the power balances in our relationship:
My partner is the working partner in our relationship. He brings the money, the wage, to allow us to live the life we have. I am the stay-at-home partner raising our little one. This is hugely important to me, my lifelong dream of how I wanted to raise my child.
Now, I could take the point of view that my partner is in a position of power over me. Without the money he brings in I could not be a SAHP. He has the power to withdraw his support of my choice, withdraw the money, and cause me to have to reenter the work force to support our little one. It would not be the end of my universe, but it would make me very unhappy. If I chose to see it this way then he has the power over me.
And he might argue that I have power over him. The thing my partner wants most in life is my happiness and my unconditional love. Those are the two things that make his life beautiful and meaningful, according to him. If I chose to withdraw my love and support I could shatter him like glass. So I have this power, in his eyes.
I said above that this is how we COULD view the power balances in our relationship. We don't think of it this way, of course. Our relationship couldn't survive without understanding and trust, and by having power struggles and power fears we would lose a lot of that. We love, we trust as best we can that we will both do the right thing for ourselves and our partner. We do our best not to live in fear.
So back to your friend/donor and power perceptions...
Yes, your friend can withdraw his donations at any time should the situation become not good for him, and I understand that the fear in that must be huge for you. But I do not feel that this necessarily puts him in a position of power unless he is using your fear to manipulate you in some way. If he is manipulating you then you can take your own power back by choosing another donor option. (Easier said than done, I know, but there are options out there).
As he said in his letter, it also feels to him that you have power over him: your current happiness depends upon his willingness to perform, and should he need or choose to stop right now he fears your disappointment and your loss of happiness in him. Perhaps he fears that if he backs out it will destroy your friendship.
Your idea of his power may not make sense to him, and his idea of your power may not make sense to you, but you are both feeling the pull of it just the same. It is affecting your relationship, and i really think that you BOTH need to be heard, equally. You need to restore the perception of equality to your relationship in both your eyes.
If you are finding it difficult or impossible to talk about the issue with one another in person without arguing or fear then I would recommend that you both start seeing a therapist who has experience in these kind of situations, both alone and together. It sounds like you need to open up a two-way line of clear, non-aggressive communication where you can both state your true feelings and concerns and both be heard by the other. Burying these kinds of feelings can only create more tension, which is the opposite of what you both need right now.