[TheDong]

It's a potentially unethical situation at the very least.

If the person running that software asks for the source code, it would be unethical to withhold it from them. However, from the arrangement you made, it would also be unethical for the person to exercise that entitlement which they previously agreed to give up. Two unethical actions don't make a right, so you're just setting yourself up for an unethical situation all around. Better to just make it clear the source code's available on request from the beginning.