The coupon is a liability, create a liability account named for the coupons due [name]
The amount in the coupon liability account is what pays for the purchase, so as an example he buys a lathe as part of this deal, use an expense transaction, list the fixed asset account for the lathe and the amount, then on the next line use the coupon due liability account and enter the same amount as a negative number, save the zero dollar transaction. The value of the fixed asset goes up, the liability account is increased by the same amount