What was the time horizon? If you have to reach 10,000 firms this year, then I can see the hesitation about taking risks, but if you have several years to try different things, then finding a cheaper way to achieve the same outcome (and still get paid the same for the results) would be quite profitable.
I say this with the full awareness that most donor contracts are a couple of years in length, not five or ten years.
Finally - a donor could just pay for higher level outcomes, without getting too concerned about attribution. If you have kids vaccinated or graduate from primary school and are willing to pay for that, it leaves the task of figuring out how to do it in the most efficient way possible up to the implementer. Again, given a long enough time horizon to experiment.