illustration (attribution, if any possible, is at the end of the article)
Quite often, it is said to "buddhist apprentices" to do their best but also to acknowledge that the karma of the other people involved is at play too. This is usually translated by "once you're clear on the purity and wholesomeness of your intention, you have to let go" i.e. not cling to the particular results your intention aimed for.
This is wise advice, but it's often misinterpreted.
It is wise not to cling: it doesn't matter how pure your intention was. What matters is the purity of your intention now, and it will manifest clarity only if no attachment unsettles the calm reflection of "reality as it is" by the mind.
But one should be mindful that "to let go" does not means "to disengage". If you had a wholesome intention but some other karmic stream neurotically interfered and prevented a wholesome result, "letting go" means… "engaging anew"!
The disruptive karma did play out; in some way, it 'cleared' and what matters going forward is how the perpetrator will appropriate the consequence, leading either to cyclical 'repetition' or to "learnt lessons".
There's new and useful information for you in how this karma played out. Moreover, you should not pretend this karma is separate from your karma… since you interact with it, it's clearly not separate!
There's an opportunity for the perpetrator to become aware of a negative pattern that just manifested, and to build the desire to get out of cyclical existence… i.e. any "bad reaction" (to a wholesome intention) embeds a potential for a wholesome insight and wholesome desire to cease dukkha.
"Letting go" in the context of a wholesome intention, associated to the bodhisattva qualities of perseverance and patience and compassion, requires to "engage anew", "engage afresh", i.e. it requires that you pay attention again and don't cling to (assume the permanency of) what you had 'understood' earlier. This is the opposite of disengagement, this is "Let go of the failure! If such an intention manifests again, just try again! The wholesome consequence of a wholesome intention is worth it." Don't cling to the old intention, but don't 'avoid' the intention either…
The buddha provided the example: did all his students 'get' the teachings immediately? Of course not! Was it because of a defect in the intention of the Buddha, or a karmic limitation of the student (clinging to prior 'certainties')? More importantly, what did the Buddha do? Stop teaching these students, or respond appropriately and adjust his discourse the next time round (now better informed of the specific difficulties of these specific students)? He didn't get upset he wasn't understood, he didn't take it personally, he didn't cling to the result of his intention to teach… but he didn't disengage either! Yes, he let the students do their part, but he kept feeding the right information (and not by merely parroting)…
photo: “Appearance/Emptiness” bronze by © Sukhi Barber, 2007 (http://www.sukhibarber.com/)