Yes, they could, although this is a commonly held teaching, and not dogmatic. As the poor souls are members of the Body of Christ, one must agree with St. Bellarmine and Suarez that it is possible and permissible to appeal to the poor souls for their intercessions.
And if a priest binds that someone is to be released from Purgatory, is God bound by the priest's declaration?
The priest does not bind anyone to Purgatory. The priest and any other soul remaining on earth can merely beg for the mercy of God for the release of these souls. The Church does not release anyone from Purgatory.
If a priest declares that Hitler's sins are forgiven, is God bound by that one too?
Jesus Christ, having all power and authority from the Father, gave the Apostles the power to forgive or bind sins. If a person makes a valid confession with contrition, that person's sins are forgiven in heaven. The priest serves as the visible means of identifying that one's sins are indeed forgiven. Thus, if Hitler came to a priest with true contrition, God would forgive that monster his sins - just as He promised He would. The ultimate sin is not murdering 6 million Jews, but in thinking that God WILL NOT or CAN NOT forgive a man's sins.
Regards
"Jesus Christ, having all power and authority from the Father, gave the Apostles the power to forgive or bind sins. If a person makes a valid confession with contrition, that person's sins are forgiven in heaven."
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I'm unfamiliar with any SCRIPTURE that indicates any supernatural powers extended beyond the Apostles.