Welcome. That is a tricky problem. Try this:
#+BEGIN_QUERY
{:title "Somme des Montants"
:query [:find (sum ?v)
:with ?b ; prevent identical values from counting only once
:where
[?b :block/properties ?p]
[(get ?p :montant) ?m]
[(* 1 ?m) ?v] ; convert ?m to actual number
]
}
#+END_QUERY
Pay attention to the two comments.