Yazid ibn 'Abdullah said, "'Abdullah ibn 'Umar sent a slave of
his with some gold - or silver - and he changed it and deferred
the exchange (i.e. he changed gold into silver or vice versa and did not
take the money straightaway. This is haram.) Then he went back to Ibn 'Umar
who gave him a painful beating. He said, 'Go and take what is mine and
do not exchange it!'"