Thanks for all advices.

The 18-year-old striker with market value 105M was sold for 126M & 4 tokens. His quality was 109, and was 1 skill point away from next star (actually 9 skill points had not assigned). He was acquired earlier in this season and gradually developed to a quite good striker but was not my most important striker. I got a replacement player with 108 quality for 1 token. I hope I was not silly to sell the young striker.