The torque is given by R x F, not F x R.
In the z-direction: N=mg. In the forward direction: ma=-uNv^=-umgv^=f, where f is the force due to friction.
The torque on the contact point is then R x f = -umgr (r^ x v^) = I dw/dt, so dw = -(5ug)/(2r) dt (r^ x v^).
If Ww^ (magnitude and direction) is the current angular momentum, the update after euler integration is [ W w^ - ((5ug)/(2r)) (r^ x v^) dt ].
There is more analysis that could be done, but check if this sign change in the cross product does anything for you.