Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Any amount of sultanas can be toxic to dogs because the active toxic ingredient is very concentrated in this fruit. To be safe, if your guy has ingested this within 2 hrs, induce vomiting by giving him 16 teaspoons of hydrogen peroxide. If he does not vomit within 15 minutes, try again. You can do this 3 times. If he has not vomited this, your vet will have an injection that will remove this from his system immediately.
The signs to watch for are vomiting and diarrhea and kidney failure a few days after ingestion...better to be safe for your guy.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta