Fold the bill up then encase it in plastic.
I especially like the idea of foreign “dollars” if you can find twenty different “dollars”–a US buck, a Canadian loonie, an Australian dollaridoo, Jamaica, HK, etc. I don’t know if there are twenty, and rounding them all up would be a right pain in the ass, but it would still be awesome.
I also like the idea of 20,000 coupons each with a cash value of 1/100 of 1 cent. Might take you a while, though.
I know a way that you can mail him his payment that would be completely untraceable to you–it would be postmarked hundreds of miles from where you live and the USPS will do it for you absolutely free and perfectly legal (and you don’t have to travel farther than your own PO). You can then include in the envelope, along with the cash (or money order’s even better), a “letter” you create (mock up some fake letterhead) telling him about a refund he got or award he won or some such. Make it look convincing, and then deny-deny-deny-deny when he asks if it came from you (tell him you’ve been putting it off or some similar excuse).
Inbox me if you want the specific details on how to do it.
Cough Not been paying much attention to the news recently, have we?
P.s. p.s. If you’re resourceful enough, you can even use my method to show a postmark on the letter to be of a date before that of the actual game you bet on! This should really confuse the issue and boggle your friend’s mind as well. 
I think the point of sending Canadian dollars was that you’d get the hassle of a foreign currency, but not actually be shorting him on the bet.
Send this filled out along with a check telling him it has been filed with the appropriate authorities.
Piker, if we really want to get into Monopoly money amounts, go with the Zimbabwean Dollar: $20 US gets $Z 612,903, at today’s exchange rate. And it’s still dollars, not Rupiah.
Twenty of an assortment of US dollar coins; Susies, Sackies, Washington, Adams…wrapped in a dayglo condom.
Go to the post office and buy $20 worth of stamps. Depending on how helpful or malicious you’re feeling, you can purchase forty-eight 41¢ commemoratives plus 39¢ worth of lower-denomination issues, or 2000 one-centers, or a $16.25 Express Mail stamp and $3.75 worth of miscellaneous definitives.
If you have more time, you can go to a stamp dealer and ask what older mint USA releases he/she has in stock. Even some pictorials from the 1930’s are available at (and sometimes below) face value. And since they’re still valid for postage, they could make interesting additions to Christmas cards or other personal correspondence.
Ah. I getcha.
I once put a $5 bill in one of those foil balloons before it was filled with helium. I gave it to my cousin’s daughter on her birthday. I told my cousin about the money but not her. A few weeks later I asked her if she got the money from the balloon but she had no idea what I was talking about, her father never told her. She said she released the balloon the next morning, she wanted to see how high it would go. A 20 would work in one of those balloons.
Encase it in Plaster of Paris. If you’re creative, you can made a mold of something in wet sand, put the bill in, pour in the plaster, let it harden, remove it from the sand and mail it off.
You could sit in wet sand to get an impression of your ass. You can send it with a note telling him to kiss it.
Fellow Okie checking in (also an OU grad). If you send me a 20, I can turn it into a luggage tag. Email is in my profile. I think.
Ask for dollar coins at a bank, and all you’re likely to get are Sacagaweas, not SBAs.
No need to go to all that trouble. Just get it laminated. Perfectly preserved, perfectly unspendable.
If you are willing to spend a bit more and have leftovers for next year, you can buy $2 bills in sheets. It is completely legal tender, but good luck spending them without the storeclerk calling the cops on you for counterfeiting!
Not quite. The Bureau of Engraving has an office that you can send damaged bills to for them to evaluate, and if they agree it’s authentic US currency, they’ll reimburse the sender for the value of the damaged bills.
Of course, it’s not exactly a quick process. 
I assume you’ve read this, right? (The story of the two brothers who gifted an ugly pair of pants back and forth for 25 years, getting more creative each year in their methods.)
My two cents: buy him 1000 shares of Enron Stock. (pun!)
Mail him a large empty manilla envelope with 20 dollars worth of stamps covering the front and back, then take it to the post office and pay for the shipping. They’ll slap a postage sticker on it, and the stamps ought to be legal for him to use later.
Double points if there happen to be Oklahama football stamps you can get.
I have a bunch of Zimbabwe dollars lying around. I could probably send you Zim$20 which as of today is worth about US$0.000683. Actually, I have a lot more Zim $50 bills than $10 bills.