The per filium
in Eastern theology concerns the Spirit's eternal manifestation or shining forth from the Father through the Son, but not His reception of existence or essence, which is from the Father alone.
When - according to the Fathers of Blachernae - theologians say: "that the Spirit proceeds 'through the Son,' what is meant here is the eternal manifestation of the Spirit by the Son, [and] not the purely [personal] emanation into being of the Spirit, which has its existence from the Father. Otherwise, this would deprive the Father from being the only cause and the only source of divinity, and would expose the theologian [i.e., St. Gregory of Nazianzus] who says 'everything the Father is said to possess, the Son, likewise, possesses except causality' as a dishonest theologian" [Council of Blachernae (AD 1285)].
I recommend reading Aristeides Papadakis's book "Crisis in Byzantinium
" for more information on this topic.
See also: The Tomos of Blachernae (AD 1285)