Rice paper can easily stick to the cel.  But it's 
also extremely easy to remove. All that needs to 
be done is to dampen the paper and it will come 
right off. Use distilled water and a plain white 
napkin or paper towel to ensure no chemicals get 
onto the cel. This is how I remove it and I've 
never had any problems.  They will want to make 
sure that the rice paper is damp all around the 
painted area so that no paint is pulled off 
though.  Good luck with the buyer, but I would 
definately not refund the money. He or she should 
be thankful that you took the time and effort to 
put rice paper behind the cel rather then risking 
having the sketch become stuck. Accusing you of 
lying is just completely out of line. =/