"buying an avocado and some mangos from a local’s stand and bringing them delight by simply not asking for change."
This is another example of Westerners who only spend a short time in Africa thinking that they are 'helping' locals but really doing the opposite for both the locals involved and other visitors who follow them or Westerns who live in Africa.
These actions whilst maybe good hearted are not well thought through and simply serve to perpetuate the idea that non African's are different from Afican's and should be treated as such. It further ingrains the idea in the local population that they deserve (and need) 'hand outs' and charity from visiting Westerners and that this is the only way they can improve their lot.
The likely scenario is that the next time a Mzungo approaches this lady selling the mangos she will be hoping that again the buyer does not ask for change and may even try to withhold the change unless he/she explicitly asks for it. She will then resent that individual and start expecting that all wazungo will pay extra for her goods without any reason. She will over time start to rely on this idea that Mzungos will pay more and will see this as her only opportunity to 'escape' her current situation.
She will remain poor due to her attitude problem.
Future Wazungo visiting Iten will be treated badly by her if they disagree with her unwritten pricing rule that white people pay extra!
Same with giving sweets to local African kids - short term excitement for the kids (which makes the visitor giving the sweets feel good about themselves) but does absolutely nothing positive for the kids beyond the 30seconds they can taste the sweet.