Consider adding what is called a "Goodwill" this is the proceeds of what is being paid for that is projected outside of traceable assets in the business. Your situation is not typical use for business goodwill but i think it could be easily applied as it certainly adds value to future earnings and acts as branding for your acquired business.
I would recommend coming to a consensus of ratio of sales:gifts then claimed:non-claimed from that gift ratio. Because goodwilll applies to branding and market recognition, assuming the gift certificates have aged, the goodwill value of the gift certs have low value to seller and high value to buyer (as if buying at discount). This benefits the buyer while seller still gets some compensation for sales value added to the business itself.

my most basic approach would be to
TA = Total Asset Value
CC = % of Agreed upon Circulating Certificates
GR = Going Market Rate
GW = Goodwill Estimate
NW = Net Worth = Assets - Liabilities

TA/NW = Assets - (Liabilities+CC)
GW = TA - CC
GR (what you pay to acquire) = TA + GW

my approach only, I am not an accountant, I simply have an MBA and have simply general good understanding of your situation.