Players/agents not knowing their value

I'm managing TNS in the Welsh Premier Division, and at the end of Season 1 an English/Turkish goalkeeper by the name of Gokhan Cicek came through the youth setup. He was already better (at 16) than my previous No 1, so became first choice almost immediately. By the Jan 2016 transfer window, bids of £30-40k were coming in from teams in the upper reaches of the English Championship & lower half of the Premier League, so I offered him a pre-contract to sign on his 17th birthday (29/7/16), which he accepted.

Fast forward to the end of the 16-17 season, and the bids were getting ridiculous - so much so that I ended up accepting a deadline day bid in August 2017 from Sheffield Wednesday (who'd just won the Championship) worth £300k up front, with additional monthly installments and clauses making a potential value of £1.2M! (I haven't mistyped that - one point two million pounds for a 17 year old 'keeper with only Welsh Premier League experience!)

He declined the contract in Yorkshire due to the wage offered, even though it was probably 50 times what I'm paying him, so he stayed at TNS for the 17/18 season. Midway through, he (or his agent) came to me demanding a new contract - I thought that was fair enough given the way he's been playing but when I entered negotiations there was a non-negotiable demand for a Minimum Fee release clause - of £5,000!

I withdrew from negotiations, but he's now entering the last year of his current deal so I really need to get him signed up to a new one, but I can't face the possibility of losing him for just £5k when I could have got £1.2M last year.

EDIT: He's now decided he doesn't want to enter talks as the rest of my squad isn't good enough to help him reach his goals. Shame he didn't show that level of ambition when Wednesday had £1.2M on the table last summer :-<

Update: Just flogged him to Sunderland. Got £425k up front, £20k/month for a year (£240k total), £35k for each of his first 10 league games + an additional £160k after all 10, and a friendly.

So a guaranteed £665k, potential additional £510k to make £1.175M, and whatever money I can make from the friendly.

Not too shabby. They finished bottom of the Prem so I'll have to keep an eye out for how he does in the Championship