Praise be to Allah.
We put this question to Shaykh Ibn ‘Uthaymeen, who replied:
What was offered to him?
We said: he was told, “You have put a lot of effort into this, so you deserve a reward.”
The Shaykh asked: Was this before the sale or afterwards?
We said: After the sale.
The Shaykh said: there is nothing wrong with it and there is no sin on him.
We said: And if it was before the sale?
The Shaykh said: That would not be permissible because then he may try to do the purchaser a favour.