Foul play by an attacker AFTER scoring a try.

[Dickie E]

and give him the YC

Having done so is he allowed to keep it as a momento?

[Taff]

After the try was awarded, I would deal with the foul play by the attacker and give him the YC and tell captain that following the conversion kick (which obviously now cannot be taken by the player YCed as he is already on his way to the naughty chair), we will restart with a PK to the non-offending team at the centre of halfway.

Works for me.

[Richard smith]

Having done so is he allowed to keep it as a momento?

NO!!! You may need it later in the game