Free goods for Basket items

Has anyone come across the following Pricing scenario:

"For a combination of 10 Qty of Item A and 5 Qty of Item B, you get 1 Qty of Item C free in the Sales order."

Defining Item A, B, C in a Basket and assigning Item C as 100% discounted adjustment in Enhanced Basket pricing is possibly not the desired solution, as Item C is possibly not known by the user at the Order entry time.

• I've defined Item A & B in Basket master with Accumulation rule as '1' and Pricing rule as '2'.
• Defined Adjustment details with level breaks at 15 with minimum Qty's as multiples of 10 & 5 in Adjustment per basket item respectively.