Well, you can't order something and then cancel it. What you can do, and I've done this many times, is buy something and return the unopened, new product with the old receipt. Then I have a valid receipt to turn in with the rebate. They do check the receipts and if you just buy and return something, they'll know and deny your rebate. The key is to get it done within the old receipt's return window.
This doesn't always work with something where they keep track of serial numbers like processors and sometimes RAM. And I've never done it with something mail order because in that case, unless the rebate is substantial, it's not worth all the extra shipping costs.
Nowadays, it takes me maybe 5 minutes to get a rebate form together if you have a multifunction or a scanner and a printer. If it takes longer, then you need to wait til you're sober.
It's worth the hassle if you can float the cash for 2-3 months.