For the customer it's simple. Either, 7 cases for the price of 6, or 6 cases for the price of 5. I think you mean the first of these two. Just make sure everyone is agreed!
As discounts, that's either a seventh off normal price (because the customer pays only 6 sevenths of normal price for every seven cases), i.e. roughly 14.3%, or else (but we don't think you mean this) a sixth off, i.e. 16.7%.
For you, it's trickier, I agree.
But your cost price is reduced by a fourteenth (7.1%), because the supplier is paying for each fourteenth bottle.
And your profit is reduced by a thirteenth (7.7%) of the old profit, and then by another 7.7%, because...
Let original cost price = C, original selling price = S, original percentage profit = M, and new percentage profit P.
And new cost price = 13 fourteenths of C, and new selling price = 6 sevenths of S.
So, your profit goes from 29.8% down to, theoretically, 19.8%.
And that figures. E.g., check your percentage profit on one case after throwing in a free bottle.
Actually, that shows that I didn't need to mention the supplier's contribution at all. Think of his bottle as going straight to the customer. That'll simplify a bit...