Listen up i had the same dilemma in my Dortmund save from the day 1 i got offered 60m for lewandowski ...
And I mean 60m from day1 of my save (plus another 20m i got as transfer budget, so around 80m totally !!) to sell one of top3 strikers in the game with an exchange of reorganizing and strengthening my whole team with that money !
Any player can be replaced sooner or later trust me.
You already got quality stikers, And if those money help for restructuring other areas of your team that are weak or give you more depth do it without second thoughts.
I know you are thinking: "Werner might become a monster striker in couple of years etc etc" but 1 player won't win you big games/trophies and Werner is not even in top 10 strikers ingame the first 3-4 years ..
On the contrary your whole team will win you those especially if they are all wonderkids and well developed.
That's what I like to do too in my save and if i hadn't sold lewa from the start i wouldn't be able to win eveything (including CL) from season 1!
Imagine half first season i had only abumeyang as starter ST that you are not even close calling him a "world class striker" (and Zivcovic as sub at the age of 16).
None else ..
i only bought Mitrovic in January 2014 to help me win everything at the end of that season..
So for me do it if the benefits of this transfer are more than just keeping 1 quality striker among 2-3 others while struggling in other areas of your team !
Ps: I saw you 've been using JK v2 tactic.
How is your testing experience of it ?
I use JK v1 in 14.1.4 patch but still works fine for me..(rotating this tactic with another great one i found)
Is JK V2 working better for you than v1?