I personally would not give him a copy of the invoice. That is your invoice, not his. If he wants a bill of sale or evidence of his purchase, then create one for him, and have him sign it and keep a copy for yourself (that he agreed to the terms). I'd find out why they want it, as it may be for tax purposes as well as warranty. I don't think there was warranty info in the box.
Make a receipt similar to this:
Sold to on this Date:______________________________________________
(items below are necessary for ownership and warranty transfer)
Name:_________________________________________________________
Address:_______________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
Phone:_________________________________________________________
Email:__________________________________________________________
One XXXXXXX (and peripherals if applicable) as described below for the amount of $_______________(cash check money order) The warranty balance will be transferred via applicable means into the new owners name as above. A copy of the applicable warranty by XXXXXX is either included here (or attached) for your convenience. The sale is as-is to the buyer from the seller, and all warranties shall be fulfilled by the manufacturer as per their terms only. The seller on this receipt does not support hardware or software issues with computers after purchase (unless agreed upon under separate terms), and the buyer agrees to this and shall not contact the seller for support. The buyer also by signature agrees that the XXXXXXX was in working and satisfactory order and condition at the time of the sale, and when relayed to the buyer.
Seller:____________________________________________________________Date:________
________________
(Print Name)____________________________________________________________
Buyer:____________________________________________________________Date:_________
_______________
(Print Name)____________________________________________________________
==========================================================================
DESCRIPTION OF ITEM:
He should be happy with that (and you may want to print of copy of the warranty from Dell's site and give it to him for his convenience and reference)...and I've never had any issues with using the receipt above.......edit as it suits your particular situation.