Is the donor sinful if he takes back his donation (fans) from a mosque under construction after those in charge of it mistreated him and diminished the value of his donation?

What the young men did, belittling what the person volunteered, is a mistake and close to the characteristics of the hypocrites who mock those who give in charity, whether little or much. A Muslim should beware of these characteristics. As for the one who gives in charity, he should be patient and know that he is dealing with Allah, and people's words will not harm him as long as his intention is sincere. Regarding the ruling on transferring waqf fans, the default is for them to remain in the mosque for which they were endowed, and it is not permissible to transfer them unless there is a [greater] benefit.