What is the ruling on disbursing money to a customer and then keeping it as a trust after a legitimate possession (al-qabd ash-shar'i) has taken place?

If the buyer takes possession of the currency in a legally valid manner, and then leaves it with the seller as a trust (amanah), there appears to be no impediment to this. However, it is not permissible to reserve gold by paying a part of its value and securing it with the merchant until the full value is paid; rather, the full price must be taken possession of.