Wiktionary
agreement in principle
n. (context idiomatic legal English) A legally enforceable, but incompletely specified, agreement between parties that identifies the fundamental terms that are intended to be or are agreed upon.
Wikipedia
Usage examples of "agreement in principle".
Obviously, no one has discussed Case Red Alpha with the Erewhonese, but last week the Erewhon Ambassador initialed an agreement in principle for a defensive military alliance with us.
Despite the tentative agreement in principle for on-site inspection, the main effort of verifying compliance on an agreement would lie with the spy satellites.