Is it permissible to take three dinars in exchange for five, with the remaining two dinars to be paid later?

If you gave your friend five dinars and took three from her, leaving two dinars with her to collect later, this is usury (riba) because exchange (sarf) must be hand-to-hand. To avoid this, have your friend exchange the five dinars for you, then you take the three, and leave the remainder with her as a trust.