SPONSO and Promissum ?

How different between SPONSO and Promissum ?

Salve efferysmiller et gratus nobis tuus adventus. Hi and welcome, efferysmiller
I look up a dictionary. Sponso is a verb (I become engaged/promised to marry), promissum is a noun (a promise). Of course, “sponso” could also be ablative or dative of “sponsus”. Sponsio -ionis is a noun similar to promissum.
In dictionarium inquiro. Unum verbum est, aliud nomen. Et nomen sponsi enim ablativo vel dativo casu est “sponso”. Sponsio nomen promisso simile est.

So what’s “i promessi sposi” (Manzoni, 1842) in Latin?

‘the promised spouses’ (the betrothed) … conjuges sponsi/ sponsi/ illi conjuges sponsi/ illi sponsi …?

Cheers,
Int