He was probably aiming to strike a deal. He didn't need the medicine himself, and he'd said as much when Sasuke confronted him about it. He must have known somehow that Sasuke would go for the medicine, meaning...

He completely fell for his trap.

He does seem a bit surprised at Heiji's observations however, clenching a hand by his side experimentally at the mention of the gash.

"...Do you think it's true, then? What he said."