I suspect you'll find plenty of folks who will deduct this as "advertising" but I'm with IRMN given this particular fact pattern.

Maybe have them change the business model?  I'm thinking something like "bring us your donation receipt to abc charity and we'll give you a xx% discount on food at our restaurant."  Then the funds aren't running through their hands, no sales tax charged on the donation part, the charity still gets paid, and deductions might be spread to folks who don't normally make contributions and can take advantage of the charity deduction for non-itemizers this year.