Your books on the income side can only reflect what you actually took in as money. In-kind can show as in-kind, but not as cash.
The store owner gets a receipt tanking them for supporting your Org., and 'donating a voucher with a retail value of $70'. What it sold for bears no value on their write off. Their own accountant will help them figure out the write-off, which is typically only wholesale value, to which you are not privy .
The person that wrote the $100 check gets a receipt thanking them for supporting your organization, noting they wrote you a check for $100, of which $30 is tax-exempt. This is because they received a $70 gift card, thus receiving a direct benefit from their donation that is not tax exempt. In continuing this train of thought, the entire $100 check gets credited to your organization.
You would not want to count the $70 gift voucher twice, as this would not provide an accurate accounting system, and you would show more profit than resides in your account. Keeping an in-kind log book is always a good idea, because you will want to use those numbers when providing annual report information, and thanking the community which supports your organization.