Is it a sin to dispose of Zakat money that one was entrusted with distributing, and what is the expiation required in this case? Is it obligatory to pay back the original amount or more? Is it permissible to purchase mosque supplies with this money?

Verse 60 of Surat At-Tawbah specifies the categories of Zakat recipients. If you are poor or fall into one of the categories of Zakat recipients, then it is permissible for you to take from the amount according to your need. However, if you are not among the Zakat recipients, then you are responsible for the mentioned amount and must return it to its rightful beneficiaries, along with repenting and seeking forgiveness, for withholding Zakat from its rightful recipients and betraying the trust. Zakat should not be spent on mosque affairs except in cases of extreme necessity and the absence of other funding, in which case it is disbursed from the share designated for "fi sabilillah" (in the way of Allah).