I wouldn’t feed that, the black spots are some kind of fungus. It might not be toxic, it could be harmless but I’m very much for “better safe than sorry” when it comes to buns.
Willow is best in spring/early summer, when the leaves are fresh and green. Later in the season they can become hosts to various types of funghi aka “rust”, especially rainy summers. Next year maybe you can get a bunch of twigs earlier in the season and dry them for later use? Willow dries very well.
If you want to give your buns a healthy green treat maybe you could grow some maize (corn) grass for them in a pot? It’s just popcorn maize that you sow in some potting soil, you dont wait for it to grow into big plants, just let it grow into grass, then cut some off now and then and feed as a treat/enrichment. Sow many seeds, more than you would if you were actually going to grow corn cobs. (Corn grass is ok for buns to eat, its the mature “fruit” i e the actual corn and husks and corn cobs that are unsafe and unhealthy for rabbits).