To a guilty mind even a gift arrives as a threat. Finding the payment returned, these brothers do not rejoice; their hearts fail, because they cannot imagine receiving anything but the reckoning they expect. So they read grace itself as menace and ask what God has done to them, certain that mercy must be a trap. This is the ego's world, where love uncondemned looks like danger and the unearned kindness only deepens the fear, since it fits no ledger the mind knows how to read.
The silver in the sack was meant as favor, but a mind braced for punishment turns favor into dread. Only the willingness to be innocent lets a gift be seen as a gift.
Money: the sense of a debt owed, which the mind expects to pay and cannot believe has been forgiven.
Sack: the mind itself, carrying what it thinks it owes wherever it goes.