Your understanding is correct. In order to make a deductible gift of salary, they must first accept the salary (as reflected on a W-2 or 1099). Otherwise it is a contributed service which is not deductible per IRS Publication 526.
However, you may certainly recognize the waiver through soft-credit or similar protocol.
John
John H. Taylor
919.816.5903 (Cell/Text)
Big Ideas; Small Keyboard