I don't see a huge problem with it being that it is for work completed and it isn't an ongoing gift that implies he's getting a gift for hitting a value.
Once, I was at a house in the country, and this guy made BBQ sauce, and he gave me a gallon jar of it. At the time, I thought nothing of it. I took it home, took it to the family BBQ. Everyone loved it. A month later the guy calls and was mad about my appraisal. I couldn't help him. I had forgotten about the gift, and I had taken it innocently, and I obviously didn't take it expecting to hit a value. The worst part was, every one wanted to more of his sauce, but he was too mad for me to call him.
Anyway, it made me think twice about taking a gift of any kind. However, if my good clients or realtors send me a gift at Christmas or in a Thank You note, as a regular part of business communication, and without expectation, I really don't see a problem. You might should make sure you are showing appreciation for his work and not the value.