you could do something like "show mw the money" make it look like $100 dollar bill or little bills all over it!! I hope this helps!!
I would make the top of the sheet cake look like a sales ticket for a car. Put his name in as the salesman, and put some really old model car on the ticket. Then put 'horrible' on the condition of the car and put a huge unheard-of price on the ticket. Then at the bottom put an large amount as his commission.
A lot of car dealerships will cut the tie of the salesperson when they make their first sale (my husband used to be in car sales and anytime he switched dealerships, I wouldn't let him wear his good ties until his first sale was made. 
If they made the sale in the morning, they'd have to walk around the rest of their tie cut midway acrost their chest -- all fraying at the ends. Maybe you can incorporate something like that?
I'd make it look like a big hunk of cheese 
(get it cheesy)... ok, really, I like the money idea - but it might be easier to do a check for some ridiculous amount of money as commission. Or make it look like an open greeting card with the following message inside..
"Congratulations on your sale....
Only 999 more to go before you get paid"
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