I’m not sure I agree with that. I think he’s absolutely free to come up with whatever ask he wants in the first place. But once he did ($50k) and once a buyer agreed to it, I’m not sure it’s fair to revisit the price.
If I list my house for $20, then realize it might be too low and raise the asking price to $100k, that doesn’t suddenly mean I’m forced to accept the first offer that comes in.