You're off the hook, buddy. When I read your OP, I thought you wanted to back out because of some personal grievance. In that case I would lean towards having to keep your word, other things being equal.
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But your objection is based on serious moral issues and possible condonement or assistance in evil. You're certainly exempt from fulfilling this.
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That being said, would an honorable man attend a church at which he cannot in good conscience donate a set of vestments? That might be something you want to consider. I'm leaning towards "No".