Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allaah preserve him, who responded by asking,
“Why did he agree to keep a box for this charity?”
(We said:) Because it was the charity that he knew of, then he found another that is more trustworthy.
(He said:) It is not permissible. So long as he collected the money with the intention of giving it to the first charity, it is not permissible for him to give it to another.