Fixer

He is an excellent fixer, and has always been regarded as such.

Source: tatoeba (304338)

The fixer removes the undeveloped silver salts from the film, rendering it no longer sensitive to light. Pour the fixer — at a temperature within plus or minus 5° F of the developer's — into the tank, using the amount specified by the tank directions. Agitate with ten inversions initially and then for 10 seconds out of every minute for the remainder of the fixing time. The fixing time is not as critical as the developing lime, but stay within the time range suggested in the fixer directions.

Source: wiktionary

A professional bank robber commented on this [a point in the thief's memoir]: 'Perhaps the author means by this that the fixer with whom he is acquainted works only on crimes not involving violence. It is true that there are specialists even in the fix line, and a man who has an in [in = advantageous position] to fix con cases might not be able to fix robbery cases. But if the author means that the fix does not exist in armed robbery, he is badly mistaken. It is merely a matter of knowing the right party to go to.'

Source: wiktionary

Yet at the same time, this is also the source of what journalists see as one of the major risks involved in dependence on the fixer: the fixer may determine, to some extent, what the journalist sees. It is at this point that differences between journalists become apparent in the interviews – they give different evaluations of the risks. Clearly, this is a point of tension in their understanding of the situation: the prime positive qualities of the fixer are also the potentially negative ones.

Source: wiktionary