Once you start negotiations with a player, the specific transfer and salary fee is out of your hands. After a few weeks of making the bid the scout will return with salary and transfer fee details if the bid is successful. There is no way to manually input the transfer fee, the fee is decided by the scout and the other club and it is up to you whether you agree with these conditions, you can negotiate with the club once the conditions have been set if you dont agree with the price.
Scouts also have different abilities so keep an eye out for that, some scouts for example are better equipped to deal with domestic players whilst some scouts are better at signing younger players.
Step 1 - start negotiations with a player
step 2 - wait for the scout to return with salary and transfer fee details
step 3 - decided whether to agree to the conditions, once the conditions have been set you are allowed to adjust the transfer and salary fee to try and knock the price down, I would avoid doing this at risk of breaking the deal.