What is the ruling on exchanging dinars for euros via bank transfer, given the absence of spot exchange (taqabud) in the same session (majlis)? And is it considered usury (riba) to sell euros at a price that varies according to the quantity, such that the price decreases as the purchased quantity increases?

It is not permissible to exchange dinars for euros through the "aforementioned trusted party" unless the exchange takes place in the session of the contract and constructive possession occurs. There is no harm in the exchange rate changing for any reason, as long as possession takes place in the session of the contract.