It's fair to say this summer's transfer window has thrown up a fair few surprises. 

The biggest so far perhaps, has been Romelu Lukaku's move to Manchester United from Everton. 

The striker cost United at least £75 million, but the surprising part of his move wasn't the cost, it was the club he chose to go to. 

For months, it was reported that the Belgian was top of Chelsea's wish list, with Roman Abramovich keen on bringing him back to Stamford Bridge.  

But the Blues waited too long to make their move and before they knew it, Lukaku was wearing red.

Another move that caught fans off guard was Kyle Walker's transfer to Manchester City. He left Tottenham for a fee of around £50 million, making him the most expensive Englishman ever.  

But neither of those moves would be quite as shocking as one that's been rumoured for a number of days.

Borussia Dortmund's Pierre-Emerick Aubameyang was one of Europe's hottest players last season, scoring 31 goals in the Bundesliga.

Earlier this summer, he was linked with a move to the Chinese Super League, but the striker turned down an offer from Tianjin Quanjin and their transfer window is now closed. 

Instead, the Gabon international has his mind set on a sensational move to the Premier League, with Chelsea leading the race for his signature. 

If Antonio Conte managed to sign Aubameyang, it would surely become one of the biggest deals of the summer.

But once again, Chelsea are stalling on making an offer, despite reports that they're keen to land the 28-year-old.

In fact, Dortmund haven't received an offer from any club and CEO Hans-Joachim Watzke has given an ultimatum to any potential suitors.

He's said that the interested clubs only have a few days to open talks with Dortmund over a transfer for Aubameyang and if they don't, they'll miss out on him should they make a future offer.

"We have not had any offers," he told Welt am Sonntag as per Goal. "We will wait a few more days but not long anymore."

"We have told Aubameyang: 'if you want to move and have a club ready to meet our conditions, come to us and we can will talk about it'. But we also explained that this is only possible within a certain window of time."

Watzke did go on to say that he'd prefer for the striker to stay in Germany, but with Aubameyang looking for a move away, that may not be possible.

"Personally, I would prefer he stay with us, but there are a few clubs on this planet where he can obviously earn more."

It appears Dortmund are willing to listen to offers for their star player, but if Chelsea are serious about signing Aubameyang, they better act quickly.