There is absolutely no way I would clean up dog poo from my lawn. And definitely not multiple piles! That's just disgusting. Leaving waste on another person's property is wrong on every level.
Having said that, I also wouldn't have my neighbor building me a fence - DH and I would whip that up ourselves in a weekend and be done with it.
So I guess it's one of those examples of different preferences and priorities. To some, the dog poo is no biggie whereas the fence building is a big deal. To me it's the opposite.
I guess I do agree in principle that when somebody is doing you a big favour then you forgive them certain things for it.
OP I hope the fence gets finished soon and you can have a poo free lawn once more! As they say, good fences make good neighbors!