I see.
Can't you just add the value before sending it to your query:
sSQL3 = "update [Inventaire] set [Quantité_distribuée] = " & val(txtQté.Text) + val(txtQté2.Text) _
& " WHERE Période = '" & ComDateDisponible.Text & "'" _
& " AND Description_du_produit = '" & Des_prod.Text & "'"