What is the ruling on keeping a sum of 1000 US dollars that was given as pocket money during a work trip to China, knowing that all other expenses were paid for?

If the money is given by agreement between the institution and the company, then there is no objection to it, as it is part of travel expenses. However, if the company gives it without an agreement, then either it is a gift due to circumstances or as a gesture of goodwill, in which case there is no harm in it, provided it does not affect the company's evaluation. Or it could be outright bribery, which is impermissible due to the prohibition of bribery.