Could co-signer kill 1st time homebuyer rebate?

[T Handy]

I'm selling a home to a young couple on Contract for Deed (Land Contract), which is eligible for 1st time homebuyer's rebate (I've had several CD buyers get their checks already, in 4-5 weeks). I sell houses for low down payments and put a short term balloon on it to capture some of the first time homebuyer's credit, which effectively adds to the down payment. One buyer's mother--a homeowner--offered to co-sign. If I put her on the actual contract, will that invalidate the rebate for the couple? Should I use a separate promissary note to include the co-signer, like some mortgage companies do?