What is the ruling on exchanging ten Dirhams for a Dinar where the payer gives more than one Dinar to weigh his due amount later?

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If one exchanges ten Dirhams for a Dinar and gives the counterparty more than one Dinar, stipulating that the excess will serve to settle his right (the remaining weight) at a later time, this is permissible. If this period of holding the excess is prolonged, the excess amount held by the recipient is considered a trust (amanah), and he bears no liability for its loss.