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Explanation: Die Verträge, auf denen die Vereinbarungen (Arrangements?) aufbauen (auf denen sie basieren) sind uneinheitlich (z.B. bei XX nicht vorhanden)
That's the sense I get from this; there are some written agreements on which certain arrangements are based, and apparently they are not on hand at XX. Based on the context you give, "vorhanden" would appear to be a better choice than "verfügbar". (If we're talking about goods and merchandise, verfügbar might be better.)