What should a person do with the surplus amount he collected from his website members to buy something specific for the website, and is it permissible for him to use it for everything related to his website?

Those who collect donations must seek permission from the donors for how the funds are spent. It is not permissible for someone to collect donations for one purpose and then spend them on another. If it is not possible to reach the donors, the funds must be spent on the closest purpose to the one for which they were collected. For example, surplus donations for building a mosque should be spent on building another mosque, not on the poor. If the funds exceed the needs of the site, then they should be spent on the site's benefit and purchasing what it needs.