Donation Transparency, Solved.

#[public]
impl Distribution {
    pub fn release_milestone_funds(&mut self, project_id: U256, milestone_id: U256) -> Result<(), Vec<u8>> {
        if msg::sender() != self.admin.get() {
            return Err("unauthorized: caller is not admin".into());
        }

        let key = Self::milestone_key(project_id, milestone_id);
        let mut milestone = self.milestones.setter(key);
        assert_eq!(milestone.status.get(), U256::from(1)); // Must be approved

        milestone.status.set(U256::from(2)); // Paid
        let beneficiary = self.projects.get(project_id).beneficiary.get();
        let amount = milestone.amount.get();

        // Cross-contract call to Treasury on Arbitrum Nitro
        let treasury = ITreasury::new(self.treasury.get());
        treasury.release_funds(Call::new(), beneficiary, amount)?;
        Ok(())
    }
}