Is it Islamically permissible to substitute the payment of gold for the dowry, in light of rising prices and marriage costs, after an initial agreement to pay fifty thousand [pounds/riyals/etc.] and having already paid ten thousand of that sum?

There is no religious impediment to not purchasing the agreed-upon gold and instead documenting its value in the list of movables, because deferring the Mahr (dowry) or a portion of it is permissible, and documenting it in the list of movables is a way of securing rights.