2 Metaphors for guarantee

would they not, on the other hand, be disposed to conduct themselves equally well as the other African slaves before mentioned, when they came to know, that they were immediately to be prepared for the reception of this great blessing, the first guarantee of which would be an immediate and living experience of better laws and better treatment?

The other guarantees they talk of are further cessions in Asia, specifically Batoum, or the occupation of Varna, or Silistria, instead of the Principalities.

2 Metaphors for  guarantee