I see on the FAQ that the character providing the helping chooses the ability. I'm not sure about this. If they are free to choose d4 and d6 traits, and regularly do so, it undermines the value of Oaths. Plus in my test fights, I saw amusing effects between the two characters with the greatest glory each impairing the other's best ability once, to increase their chance of getting the final blow. I think I'd enjoy seeing players do that. There'd be an interesting tension: players would want to impair abilities that are useful to them, and would not want to upset the people they are borrowing the die from except when it's vitally important - and those moments will be fun.

Of course, people would then be able to do this abusively - but I think that with a functional group, the players would over a few sessions find a fun balance somehwere between helping each other and being bastards to each other.

I might have a rule of First Refusal: the helper can refuse the one ability per Helping; so if I call in an oath and demand you use Spear, you can say no, and force me to pick an another ability. Whatever that ability is, will have to be used.

Consider this:

"You owe me an oath. I need help. Pay up."
"Okay. I'll give you my d10 die, but you have to cancel two of my oaths with you."
"No way."
"Okay, then you get this d4, sucka."

Oath bargaining is a subtle art. :)

That's an amusing example, but my gut reaction is I don't think my players will like that. It'll result in the cheaping of oaths, and they'll stop being used.
They'll be happy to work within strict, set limits - like using Positioning to mess each other up, or impair high dice, and the like, and enjoy it. But I strongly suspect that the kind of bargaining you illustrate there will cause genuine ill-feelings and resentment among my players.

Okay, look at it this way.

Let's say I have 3 Oaths from you. I'm in a bind. I need help. I call in one of your oaths. You give me a d4. I'm going to be like, "OK. That doesn't help me." So I'll call in another one. Will you give me another d4? (assuming you have one). Because if you do, you know I'm going to call in that third oath. There, I just zeroed out 2-3 of your abilities. That's probably not the smartest play in the world.

On the other hand, if you actually do give me a useful die the first time I ask, I probably won't keep calling oaths from you -- I'll want to save them for when I need them. If a player creates a reputation as someone who provides good help from oaths, then that player creates leverage for herself in play. Their oaths carry value based on how helpful they actually are. The guy who always passes over his crappiest dice when paying oaths does not make himself an attractive player in the oath economy.